When your almost 3 year old is playing with blocks and starts making sets made of three blocks that he tells you are carbon dioxide molecules. One large block with two smaller blocks in each set.

...or when he asks what a spiral staircase is and after you try to explain it as simply as possible as stairs that are shaped as if they have been twisted into a spiral, he responds, "Oh, like DNA. Okay."

Thanks They Might Be Giants...