One of my boys, when he was about 3 years old, said something to his older brother, then about 5, about being selfish for not sharing his toys.
My 5 year old said, "You don't even know what 'selfish' means!" to which my 3 year old replied, "Sure I do, it's kinda like a blowfish."
My oldest also had this classic when he was about 4 years old...
One day, out of the blue, he said,
"Sometimes my weiner has a bone in it!"
Of course, while my wife and I were rolling on the floor laughing ourselves to death, he just sort of stared at us with a bewildered look on his face. Finally, he started laughing as well, though he was not sure why!