Nuts!

[rebbonk]

A farmer gets on a crowded train, takes a seat and places a large cotton bag beside him on the only other vacant seat. An unseated passenger glares at him, but decides just to sit down on top of the bag.

Suddenly the farmer shouts: "Oi! Watch out for the nuts!"

"Why? Have you got nuts in that bag?"

"No," says the farmer.

Spoiler:

"Barbed wire."