A scientist found, to his great surprise, that he was lactose intolerant (unable to digest milk sugar). At dinner that night with his two young daughters (age 9 and 4 years), he mentioned that he had found out that he was lactose intolerant and tried to explain to them what that meant.

A couple of months later, he took the kids to a local restaurant for a quick breakfast before shopping. The place was very busy, but the quality of the food and service were obviously not up to par. When they finally got their breakfast, his youngest daughter took a look at her father's omelet and burnt toast and declared very loudly to the waitress "My Daddy can't eat that toast, he is black toast intolerant."