Men!

In a train compartment, there are three men and a ravishing young girl. The four passengers join in conversation, which very soon turns to the erotic.

Then, the young girl proposes "If each of you will give me $1, I will show you my legs".

The men, charmed by this young girl, all pull a buck out of their wallet. And then the girl pulls us her dress a bit to show her legs.

Then she says "If each of you gentlemen will give me $10, I'll show you my thighs" and men being what they are, they all pull out a $10 bill.

The girl pulls up her dress all the way to her legs in full.

Conversation continues, and the men, a bit excited, have all taken off their coats.

Then the young girl says "If you will give me $100, I will show you where I was operated on for appendicitis".

All three fork over the money. The girl then turns to the window and points outside at a building they're passing.

Spoiler:

"See there in the distance. That's the hospital where I had it done!"