Two fathers and their two sons go fishing together. They each catch one fish to take home with them. They do not lose any fish, and yet when they arrive at home they only have three fish. How can this be?
There are just three people. A grandfather, his son, and his grandson.
Why this works
This riddle plays with the relationships between the characters involved. At first glance, it seems like there are four individuals: two fathers and two sons. However, the clever twist is that the group consists of just three people: a grandfather (who is a father), his son (who is also a father), and his son (the grandson). Thus, while there are indeed two fathers and two sons, they are represented in just three individuals, which explains why they only brought home three fish!