It began with a coffee cup. I have two identical coffee cups with Spider-Man on one side and the Hulk on the other. Leo saw one of the cups nearly every day from the time he was born. In his first year, when he was beginning to talk, I taught him how to say, "Hulk" because I thought it would be cute. It was. He soon naturally learned to identify the Hulk on posters around the house. It wasn't long before I taught him to say approximations of "Spider-Man", "Iron Man", and others.
As of this writing, Leo owns 5 shirts that feature the Hulk and/or other Marvel characters, and when getting dressed in the morning, Leo will often request, "Hulk" or "Iron Man". He can ID various versions of the characters in diverse art styles from the '60's and '70's to modern painted versions. Although he's never seen them on-screen, he can get as excited about seeing them in print as he does with Sesame Street characters.
I never wanted all this to happen and I'm not sure how I feel about it. I'm concerned he'll grow out of comic book characters before he has a chance to really know them. On the other hand, a thing does not have to be important to my kids just because it is important to me.