A few weeks ago I started introducing the concept of "genus" to Elias. After only one or two examples ("animals" and "furniture"), he got it, to my amazement. Next he was telling me that the genus of his Legos is "toys."
The other night I was reading to both of the kids and brought up another example. That's when the smallest boo-boo decided that "genus" sounds like "penis." Things don't always go as planned with children, do they?
My purpose is to teach the methodology of figuring out explicit definitions. I can't think of any good reason we don't learn to habitually define our concepts from a very young age, though I'm sure many non-Objectivist parents would cringe (or scowl) at the thought. Part of Ayn Rand's genius must have been that she grasped the value of this, and did it, from a very young age.