* Refraction: The water in the tank acts like a giant magnifying glass, bending the light rays as they pass through it. This creates a distorted image of the cup at the bottom, making it appear larger and closer to the surface than it actually is. This visual trick makes it difficult to accurately aim the penny.
* Depth Perception: Our brains are not great at judging distances underwater. The water's refractive properties again play a role, making it harder to gauge how far the cup is from the surface and how much force to use.
* Water Resistance: The penny has to travel through a medium (water) that is much denser than air. This creates resistance, slowing the penny's descent and making it difficult to accurately predict its trajectory.
* Swaying Water: Even if the water appears still, it's likely moving slightly. This motion can disrupt the penny's path, making it even more challenging to hit the target.
In essence, the combination of visual distortions, depth perception challenges, water resistance, and potential water movement makes dropping a penny into a small cup at the bottom of a large fish tank a surprisingly tricky task.