Why is there a heart at the bottom of my coffee mug?
Answer 1
Suppose a light source is at the mug's rim. Then, each light ray will reflect off the side of the mug, and the reflected ray will be tangent to a heart-shaped curve called a cardioid. These tangent lines will accumulate making a brighter pattern in the shape of a heart.
Move the blue point to see the tangency.
(Scroll down to see a different answer.)
Answer 2
More realistically, the light source (like the sun) is far enough away that the light rays come in approximately parallel. Then, each light ray will reflect off the side of the mug, and the reflected ray will be tangent to a curve with two cusps called a nephroid. These tangent lines will accumulate along half of the nephroid, making a heart-shaped pattern.
Move the blue point to see the tangencies.