Why does a bike rest on its leg?
Q: Why does a bike rest on its leg?
A: Because it is too tired.
This is definitely a Dad joke - It is also a riddle in the standard Q and A format, and a pun as well.
The humor in this joke centers around the fact that the phrase "too tired" sounds exactly like "two tired", and the first means "more tired than is necessary", while the second means "has two circular rubber things on the edge of its wheels"
This ambiguity is the reason why this joke is funny.