"May I touch your belly?"
Your belly may be big and out there for all to see, but that doesn't make it public property. Thankfully, some people have enough good sense to know that it's never okay to grope someone -- at least not without asking first. If you prefer not to be rubbed like a Buddha, say so. "Being polite doesn't mean letting others treat you in ways that make you uncomfortable," says Leah Ingram, author of The Everything Etiquette Book: A Modern-Day Guide to Good Manners.
Stop it by: Using a bit of humor. Try "Please look, but don't touch." Or turn the tables and say, "You can touch my tummy if I can touch yours."