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I encountered this lightbulb joke today, but I can’t figure out why it is funny:

Q: How many flies does it take to screw in a light bulb?
A: Two, but I don't know how they got in there.

My best guess is that this is a play on the meaning of the word “screw”, suggesting that the flies are flying in circles inside the bulb, but I could not find this meaning in the dictionary.

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    The joke does play on the two meanings of the word screw. But there is another detail, which adds to the fun. Flies do have a tendency to find their way inside places that are supposedly shut. I for one have often found flies inside a lamp when removing the cover to replace the bulb. I think anybody who experienced that will find the I don't know how they got in there part funny. Of course it would be impressive if a fly actually did manage to get inside a light bulb given that it is designed to not even let air get in or out. – kasperd May 24 '15 at 12:32
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Yes, this is indeed a play on the meaning of screw. The second meaning being this one, from your dictionary:

3.1 [no object] (Of a couple) have sexual intercourse.

So the questioner is asking how many flies it takes to put a light bulb in place, but the answerer treats the question as if it were asking how many flies it takes to copulate inside a light bulb.

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    I liked fornicate better. – Catija May 22 '15 at 9:23
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    @Catija I think the problem with “fornicate” was that it implied that the flies in question are not married, while “copulate” does not. (We don’t know indeed!) – kirelagin May 22 '15 at 9:28
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    Indeed, @kirelagin is right. Not all screwing is fornication. (And in the eyes of some, not all fornication is screwing!) – oerkelens May 22 '15 at 9:30
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    TIL Fornicate specifically refers to sex between two unmarried individuals. – Doc May 24 '15 at 4:28
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    @Sean No, not according to a strict reading of the definition at least. merriam-webster.com/dictionary/fornication – Doc Oct 15 at 5:12
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The joke is due to two possible interpretations of the sentence (its parsing):

How many flies does it take to screw in a light bulb?

Means: How many flies are required to put the light bulb into the socket?

How many flies does it take to screw in a light bulb?

Means: How many flies should be in a light bulb so that there is some sexual intercourse going on in said bulb?

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    Although not the accepted answer, this is the only one which actually explains both doubles of the entendre – Mawg May 22 '15 at 16:06
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    One other aspect of the joke that might be lost on some non-native speakers is the abundance of existing light bulb jokes, like this one: How many archeologists does it take to change a light bulb? (A: Four. One to change the bulb, and three to debate about the age of the old one.) There are hundreds of these. Since most folks will have heard these jokes many times, most will initially assume the asker is asking the former question, not the latter question. – J.R. May 23 '15 at 12:42
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The joke relies on the juxtaposition of the dual meanings of "screw":

  • Screw in a lightbulb - To ensure a lightbulb is securely located within its holder so that it works as a lightbulb

  • Screw in[side] a lightbulb - to copulate inside a lightbulb

So it subverts the classic "How many [x] does it take to change a lightbulb" joke with the punchline being that the copulating flies have no idea how they came to be within the lighbulb, which is the premise of the joke in the first place.

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    Actually, it is the joke teller who does not know how they got in the bulb. If the flies know, they're not telling - but then again, no one asked them. – WhatRoughBeast May 23 '15 at 1:53

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