There are lots of posts about the difference between extend and expand, but I can hardly find any posts explaining their synonymous usages. When do you use them interchangeably?
The word "extend" is used for linear things.
The word "expand" is used for two-dimensional or three-dimensional objects.
Examples of "extend":
- We can extend the rope with a carabiner
- We can extend the life of your pet dog for at most 6 months (time is modeled as being linear)
- We can extend the shelf life of our company's breakfast cereal by adding tri-sodium phosphate to it (time is modeled as being linear)
- John extended his arm to William (human arms are long an skinny, approximately linear)
Examples of "expand":
- China expanded into Tibet in the year 1959. (China looks like a two-dimensional blob on most world maps)
- The balloon expanded as the clown used a pump to put more air into it ( a balloon is three-dimensional, and has volume)