You're right to be curious! It seems counterintuitive, but even when the sun is directly overhead (at its zenith), upright objects still cast shadows. Here's why:

* The Earth is a sphere: The sun's rays hit the Earth at an angle, even when it's at its zenith. Since the Earth is curved, the rays that hit the top of an object are slightly angled compared to the rays hitting the base of the object.

* Light travels in straight lines: This angle, though subtle, causes the top of the object to be in shadow while the base receives direct sunlight.

Think of it this way: even a perfectly flat surface would technically cast a shadow when the sun is at its zenith, because the light would be hitting the surface at a slight angle.

Here's an analogy:

Imagine you're standing in front of a tall building. Even if you are directly in front of the building, your shadow will still be visible on the ground because your body is blocking the sunlight. The same principle applies to objects in relation to the sun, even at its zenith.

The shadow at the zenith might be very small and faint, but it's still there!