When Niall's class went on a field trip to the Pacific Science Center last week, he was impressed to find out how two people as closely related as mother and son (his friend Colin and his mom) could end up at nearly opposite ends of the "gene tree" simply because of a distinction like curly vs. straight hair.
One of the other differentiators was straight vs. hitchhiker thumb. I showed the kids how my thumb isn't even as long as a regular straight thumb, nevermind a hitchhiker thumb (I'll add a photo sometime). That got us started comparing thumbs among the kids and with me. None of us have hitchhiker thumbs, that much is for sure.