You know - you are there in the middle of the mat and you call uke to show a technique... And he does not have the foggiest idea what you will do. You present one side or the other, you may call the attack. But he is blank. And this first time is the real deal. Second time he already knows the technique and he will play along his habits and preconceived ideas. But this first time... With good uke you can do the technique for real. This is the little spark of joy in the darkness of responsibility of a teacher.