I dunno about lesson. But it is in physical reality. If someone asked you, "you have X number of things, so how many do you have" the (wish I knew the proper term) You have is referring to the thing that was just said.
Maybe that the thing it reminds you of is a lesson. This one is designed to be a 'trick question' when based on common English speak, the trick isn't entirely right.
You have the animals, thus when asked how many you have, you'd technically include the legs of the animals, and the legs you yourself have.
Much more a lesson than a trick. This is very similar to a word problem designed to illustrate how to recognize and discard information extraneous to a question. Remember the context - this isn't in a legal community, the post is referencing physical reality.