The riddle
What Has No Beginning, Middle or End?
A doughnut is a ring
Go around a doughnut and there is no place where the trip starts or stops. A line segment has two ends, but a closed loop has none. The word middle is the clever part, because a doughnut has a hole where its middle would be. Merriam-Webster spells the treat "doughnut" first and lists "donut" as a variant, so either spelling is correct.
A circle also fits
Many tellers answer a circle, and that answer is just as reasonable. A circle is a closed curve, so it has no beginning and no end, and its center is a point that is not part of the curve. Either answer fits the wording. The doughnut is the more playful one, and the circle the more geometric.
The solid shape
Mathematicians call the surface of a doughnut a torus, from the Latin word for a bulge or a cushion. Merriam-Webster defines a torus as a doughnut-shaped surface. A torus is what you get by spinning a circle around a line that does not touch it, and it is the standard example of a shape with a hole running through it. Britannica's entry on topological equivalence notes that a doughnut and a coffee cup with a handle count as the same shape to a topologist, since each has exactly one hole.
Questions to ask next
After the answer, ask where a ring begins. Most groups point at a spot and then realize any spot would do. With a real doughnut in hand, the missing middle is the first thing the group notices. The same round shape sits behind another riddle on the site, listed below.