My boys are almost exactly 11 months apart, and being the nut that I am I usually try to dress them the same. Aaron (younger) is big for his age and Peter is a little small for his age; so around the time Aaron was about 10 or 11 months people naturally started asking me if they are twins. Every single time we are in public.
It has become so frequent, that before I can respond Peter will say, "No, we are 11 months apart."
He has actually gotten sort of obsessed with how far apart people are in age. He will tell me, "Mommy, you and Aunt K are 3 years apart, and Daddy and Uncle Don are 2 years apart." He will also randomly ask me how far apart I am from other people.
Today, Peter and Aaron were playing with cars and Peter picked up two cars that looked identical to one another. He handed one to Aaron and took one for himself, then he told Aaron, "You can play with that one and I will play with this one. But they aren't twins, they are 11 months apart."