4 added 13 characters in body
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The main distinction, though it is somewhat blurred, is that at is used mainly with activities requiring our physical attendance and participation, and in is used mainly with domains or areas or circumstances or environments.

We can be good at ping pong, guessing games, or calming a frightened child.

Someone (or something) can be good in the kitchen, in delicate situations, in the board room, in chemistry, in times of political unrest.

The camel is good in the desert.

Stainless steel is good in the salt-air.

The virtues of the camel and the stainless steel are manifest in those particular environments.

Some activities can be understood in either manner. When we say

She is good at math

we are presenting her as someone who can solve number problems, or who offers answers in algebra class, that is, "math" is presented as an activity in which she participates.

And when we say

She is good in math

we are presenting math as a domain of knowledge, an area of intellectual pursuit.

P.S. Good means "possessing some skill or virtue", not "showing mastery" or "possessing unparalleled excellence". So it would be comical to say

Albert Einstein was good at physics.

Beethoven was good at music.

Pelé was good at soccer.

We can be "amazing","terrible", "fantastic", "horrible", "so-so", "excellent", "very good" or "not very good", or even "a genius" at or in something.

The main distinction, though it is somewhat blurred, is that at is used mainly with activities requiring our physical attendance and participation, and in is used mainly with domains or areas or circumstances or environments.

We can be good at ping pong, guessing games, or calming a frightened child.

Someone (or something) can be good in the kitchen, in delicate situations, in the board room, in chemistry, in times of political unrest.

The camel is good in the desert.

Stainless steel is good in the salt-air.

The virtues of the camel and the stainless steel are manifest in those particular environments.

Some activities can be understood in either manner. When we say

She is good at math

we are presenting her as someone who can solve number problems, or who offers answers in algebra class, that is, "math" is presented as an activity in which she participates.

And when we say

She is good in math

we are presenting math as a domain of knowledge, an area of intellectual pursuit.

The main distinction, though it is somewhat blurred, is that at is used mainly with activities requiring our physical attendance and participation, and in is used mainly with domains or areas or circumstances or environments.

We can be good at ping pong, guessing games, or calming a frightened child.

Someone (or something) can be good in the kitchen, in delicate situations, in the board room, in chemistry, in times of political unrest.

The camel is good in the desert.

Stainless steel is good in the salt-air.

The virtues of the camel and the stainless steel are manifest in those particular environments.

Some activities can be understood in either manner. When we say

She is good at math

we are presenting her as someone who can solve number problems, or who offers answers in algebra class, that is, "math" is presented as an activity in which she participates.

And when we say

She is good in math

we are presenting math as a domain of knowledge, an area of intellectual pursuit.

P.S. Good means "possessing some skill or virtue", not "showing mastery" or "possessing unparalleled excellence". So it would be comical to say

Albert Einstein was good at physics.

Beethoven was good at music.

Pelé was good at soccer.

We can be "amazing","terrible", "fantastic", "horrible", "so-so", "excellent", "very good" or "not very good", or even "a genius" at or in something.

3 added 13 characters in body
source | link

The main distinction, though it is somewhat blurred, is that at is used mainly with activities requiring our physical attendance and participation, and in is used mainly with domains or areas or circumstances or environments.

We can be good at ping pong, guessing games, or calming a frightened child.

Someone (or something) can be good in the kitchen, in delicate situations, in the board room, in chemistry, in times of political unrest.

The camel is good in the desert.

Stainless steel is good in the salt-air.

The virtues of the camel and the stainless steel are manifest in those particular environments.

Some activities can be understood in either manner. When we say

She is good at math

we are presenting her as someone who can solve number problems, or who offers answers in algebra class, that is, "math" is presented as an activity in which she participates.

And when we say

She is good in math

we are presenting math as a domain of knowledge, an area of intellectual pursuit.

The main distinction, though it is somewhat blurred, is that at is used mainly with activities requiring our physical attendance and participation, and in is used mainly with domains or areas or circumstances or environments.

We can be good at ping pong, guessing games, or calming a frightened child.

Someone (or something) can be good in the kitchen, in delicate situations, in the board room, in chemistry, in times of political unrest.

The camel is good in the desert.

Stainless steel is good in the salt-air.

The virtues of the camel and the stainless steel are manifest in those particular environments.

Some activities can be understood in either manner. When we say

She is good at math

we are presenting her as someone who can solve number problems, or who offers answers in algebra class, that is, "math" is presented as an activity in which she participates.

And when we say

She is good in math

we are presenting math as a domain, an area of intellectual pursuit.

The main distinction, though it is somewhat blurred, is that at is used mainly with activities requiring our physical attendance and participation, and in is used mainly with domains or areas or circumstances or environments.

We can be good at ping pong, guessing games, or calming a frightened child.

Someone (or something) can be good in the kitchen, in delicate situations, in the board room, in chemistry, in times of political unrest.

The camel is good in the desert.

Stainless steel is good in the salt-air.

The virtues of the camel and the stainless steel are manifest in those particular environments.

Some activities can be understood in either manner. When we say

She is good at math

we are presenting her as someone who can solve number problems, or who offers answers in algebra class, that is, "math" is presented as an activity in which she participates.

And when we say

She is good in math

we are presenting math as a domain of knowledge, an area of intellectual pursuit.

2 added 30 characters in body
source | link

The main distinction, though it is somewhat blurred, is that at is used mainly with activities requiring our physical attendance and participation, and in is used mainly with domains or areas or circumstances or environments.

We can be good at ping pong, guessing games, or calming a frightened child.

Someone (or something) can be good in the kitchen, in delicate situations, in the board room, in chemistry, in times of political unrest.

The camel is good in the desert.

Stainless steel is good in the salt-air.

The virtues of the camel and the stainless steel are manifest in those particular environments.

Some activities can be understood in either manner. When we say

She is good at math

we are presenting her as someone who can solve number problems, or who offers answers in algebra class, that is, "math" is presented as an activity in which she participates.

And when we say

She is good in math

we are presenting math as a domain, an area of intellectual pursuit.

The main distinction, though it is somewhat blurred, is that at is used mainly with activities requiring our physical attendance and participation, and in is used mainly with domains or areas or circumstances.

We can be good at ping pong, guessing games, or calming a frightened child.

Someone (or something) can be good in the kitchen, in delicate situations, in the board room, in chemistry, in times of political unrest.

The camel is good in the desert.

Some activities can be understood in either manner. When we say

She is good at math

we are presenting her as someone who can solve problems, or who offers answers in class, that is, "math" is presented as an activity in which she participates.

And when we say

She is good in math

we are presenting math as a domain, an area of intellectual pursuit.

The main distinction, though it is somewhat blurred, is that at is used mainly with activities requiring our physical attendance and participation, and in is used mainly with domains or areas or circumstances or environments.

We can be good at ping pong, guessing games, or calming a frightened child.

Someone (or something) can be good in the kitchen, in delicate situations, in the board room, in chemistry, in times of political unrest.

The camel is good in the desert.

Stainless steel is good in the salt-air.

The virtues of the camel and the stainless steel are manifest in those particular environments.

Some activities can be understood in either manner. When we say

She is good at math

we are presenting her as someone who can solve number problems, or who offers answers in algebra class, that is, "math" is presented as an activity in which she participates.

And when we say

She is good in math

we are presenting math as a domain, an area of intellectual pursuit.

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