We are tempted to assume that it s the grandchild, since the grandchild is the one who is in the learning position, and the grandparent is a kind of living Sinai. That s true, of course, but Rabbi Yehudah is actually making a deeper point, one we might not so readily assume. He is claiming that the experience of teaching is revelatory of the grandparent, too. When the grandparent passes knowledge on to the next generation, it s as if he is standing at Sinai receiving the Torah himself. In order to teach you that just as a small tree can set a big tree on fire, so do the less learned spark the minds of the more learned.
Revelation is, by definition, the meeting point between two parties.
It is a particularly intimate meeting, as the name implies. We reveal ourselves to the other in hopes of strengthening the connection between us. Literacy tutoring is revelatory in just this way. And, as is with all intimate relationships, it eventually becomes unclear who is doing the giving and who, the receiving. There is giving and receiving no both sides.