Timbre
( I'm only typing this to fulfill the character requirement)
Is this a genuine question or a curious question?
Answer:
When the cup is filled with water, the penny disappears. This happens because of refraction. When light bounces off of an object, it reaches our eyes and we see the object. As light travels through the sides of the glass and the water, it's refracted and never reaches our eyes, which makes the penny seems to disappear.
Explanation:
When the cup is filled with water, the penny disappears.
