Why can concave surfaces look convex when viewed from overhead (and vice versa)?

Mountain or valley? Concave or convex? Why is it so difficult to tell the difference when you’re viewing a surface from directly overhead? This short animation from MinutePhysics details how this perceptual illusion is explained by our natural conditioning to the Earth’s most powerful light source – the Sun – which has trained us to perceive light as always being on top and shadows as always being on the bottom.