Yesterday at karate, we did a basic warm-up (about 15 minutes). I spent the next sixty minutes teaching a very small child the finer points of stance and turning. Well, they weren't all "finer points". We started with "please, A, bend your knees."
15 minutes before class was dismissed, the assistant sensei (who was leading the class) gave us two tricky series of kicks, and told us to practice. Ten minutes of strenuous kicking. And obviously, those of us who had been teaching small ones were no longer "warm".