The other day I had a terrible day at work (I'm having a lot of those this school year), and when I came home, my daughter told me a joke she'd read online somewhere. "My grandfather has the heart of a lion, and a life-time ban from the zoo."
It reminded me of a headline I once saw on Fark.com about a memorial service for a man killed by a bear. "Man killed by bear had tender heart say friends, family, bear."
Related only because it also makes me laugh a lot is this joke: a man walks into a psychiatrist's office completely naked and wrapped in cellophane, and the psychiatrist says, "I can clearly see your nuts."
Finally, this bon mot that makes me laugh: in theory there's no difference between theory and practice, but in practice there is.