If fruit flies like a banana does that mean that time flies like an arrow?
Ahhh...
'Time flies' metaphorically implys that time goes quickly, an arrow practically flies through the air, and the species fruit flies like to eat bananas. The play is on the use of the word 'flies' in it's different uses. But put together, the two sentences separated by a comma, give the sentence a different meaning, because our brains see the meaning of the first clause of the sentence in the second clause, we see the sentence as meaning that time flies in the same manner as an arrow flies through the air (possibly direct and piercing), and fruit e.g. generic term for apples, pears, oranges fly through the air in the same way as a banana would fly through the air (possibly like a boomerang due to its shape).