There's a classic Calvin and Hobbes cartoon of Calvin at the zoo, grabbing hold of some woman's legs and being astonished when it's not his mother because all mom's look the same from the knees down.
I had first-hand experience with this when we went to the Science Museum a couple of weeks ago. We were with Kristin and her boys. At one point Xander didn't see me and ran over and grabbed hold of Kristin's legs. At first, I thought since he couldn't find me, he figured since he knew her, she was a good substitute.
The expression on his face when he looked up and realized she wasn't me was priceless. Such astonishment!
10 minutes later he did again with a complete stranger.