1. Teach in the Interstices
This might not work with really young or stupid kids, or if they’ve already been permanently damaged by a philistine upbringing, but it’s worth a try.
Kids love to get you off topic and they are never more attentive than when they've succeeded in deflecting you onto a digression. These occasions are utterly prime for teaching them something valuable, so you should always have something ready. For example, you could teach them a word like "interstices."
Write the word on the board and ask if anyone knows what it means.