Torah Temimah points to the baraisa that one who teaches his grandson is credited as having taught him, his son, and his grandson to the end of all generations (Kiddushin 30a).
English: It was taught: “And make them known to your children and to your children’s children” — this teaches that anyone who teaches his grandson Torah — Scripture accounts it to him as if he taught it to him, to his son, and to his grandson until the end of all generations.
On the daf / in context: The phrase “and to your children’s children” expands the credit of a single act of teaching into a multi-generational chain.
Type of Learning
Custom (expansive reading of “u-livnei vanecha”).
What Is Learned
Teaching a single grandson is regarded as having transmitted Torah through every subsequent generation.