What does "kill two birds with one stone" mean — and why is it funny?

informal

Meaning

To solve two problems or achieve two goals with a single action.

Where it comes from

An old proverb, in use since the 1600s, drawn from the unlikely luck of one thrown stone bringing down two birds at once.

Why it is funny

The mild humor is the wild optimism of the image. Hitting even one bird with a thrown stone is hard; the phrase cheerfully assumes you will get two, making efficiency sound like a tiny miracle.

Used in a sentence

"I'll visit the bank and the post office together and kill two birds with one stone."