My 2 year old daughter has a dirty sense of humour. Cases in point:
She burps loudly, then holds her hand in front of her mouth, gives a little giggle and states in an exaggerated voice, "Pardon me!"
I'm changing her nappy upstairs in her room. Her father returns home. He shouts up, "Hello Little Planet!" She shouts back down, "Hello Daddy!" He says, "What are you doing?" She replies, "I done a poo poo!"
This time her daddy is changing her nappy. She lets rip a loud fart and giggles wildly. Then she puts up her finger and says, "Here comes another one!" and surely a few moments later she lets rip another fart.
We neither burp nor fart in front of her (not even in front of each other really - need to keep the romance alive!) so these habits are hers alone :-)