The riddle
What Do You Call a Sleeping Bull?
Bull plus doze
Say the answer slowly and it comes apart in the middle: bull, doze, er. A bull is a male of the cattle family, and to doze is to sleep lightly. Put the ending -er on a verb, as in runner or sleeper, and the word names whoever does the action. A bull that dozes is a bull-dozer, which is also the name of a heavy machine, and that second meaning is the whole joke.
What the machine really is
A bulldozer is a powerful tractor with a wide blade across the front, built to push earth and rubble aside. Dictionaries do not trace its name to sleeping, and Dictionary.com calls the origin of the word unclear, so the riddle works purely by sound. Two meanings that meet in one spelling by accident are exactly what a pun is made from.
Getting the pause right
This one lands best when the question is asked slowly, with a beat after "sleeping". Listeners often reach for an animal word or a sleep word, such as nap or snooze, and miss that the answer is a machine. If nobody gets it, repeat the last part as "bull-doze-er" and wait.
Cousins in the same shape
Questions that open this way make a family of riddles whose answer is an ordinary word matching a description. A close relative is the riddle about a bear with no teeth, and another plays with letters in the riddle about a fish with no eyes.