At a four-legged table there is 1 grandpa, 2 dads, 2 sons and a grandson. How many legs are under the table?

6 legs

Explanation:

1 grandpa: the grandpa is the father (dad number 1) of one of those seated.

Son number 1: The grandpa has a son.

Son number 2: The grandpa's son also has a son.

This makes him a dad (Dad number 2).

The son of the grandpa's child is the grandpa's grandson.

We can say from the above, a man (grandpa) has a Son and a grandson.

Each person has two legs.

For 3 persons, we would have = 2*3 = 6legs

Therefore a 4 legged table with 1 grandpa, 2 dads, 2 sons and a grandson have 6 legs under the table.