This is how I know I'm immature: every time anyone says the word "balls," I laugh. Every time. A doctor could tell me I have 24 hours to live because viciously aggressive, tiny balls of cancer are ravaging all my vital organs, and I'd fucking laugh. I swear. I can't help it.
Also, any time grooming salons or nail places have a sign in the window that reads, "Facials," . . . I laugh.
Any time someone's name is Richard, I have to call him Dick. (If he's big and strong and unfriendly, I may only do this in my head.)
Whenever anyone tests a microphone and says, "testing," I have to say, "testes."
Other words that elicit involuntary laughter every time I hear them include "box," "69," and "knob." I'm sure there are more, but you get the gist...