Floating Coin

Generally, when the coin is dropped randomly, it sinks since the coins is denser than that of water. However, when the coins is placed gently being parallel $(\theta =0^{\circ})$ to the surface of water, it floats. This is because of the peculiar nature of water called surface tension which acts as the stretched membrane.

As soon as the coins is placed gently it creates a small depression on the water surface due to which surface tension force $F_T$ come come into play vertically upward which balances the entire weight of the coin. i.e, $\begin{aligned} F_{T}\cos\theta = mg\implies \gamma (2\pi r) = mg\implies2\pi r \gamma = mg\end{aligned}$ where $\gamma$ is surface tension of water.

If you drop the coin into the water, the gravity causes it to break the surface tension. But if you put it gently parallel to the bottom, there isn't as much initial downward force and the surface tension doesn't break.

This isn't actually much of a solution, but I want to say this before someone goes and tries to do that. This does not work for all coins. If you do it with a US quarter it will sink no matter what because it weighs so much. I know that it will work with a Japanese 1 yen coin if you are slow and careful. This works because in the creation of the 1 yen coin, they aerate the inside of the coin so that they don't have to put as many resources into the coin itself. I don't know if there are any other coins that currently exist that you can do this with, but I don't believe that there are any US coins you can do it with.

When the coin is put into the water directly it have enough momentum to overcome the surrounding tension of the water and when it penetrates inside the water it stinks downwards because of its high density but when it is put slowly it's momentum is low due to less relative speed and thus the surface tension which is the force which makes the surface like a stretched membrane which keeps the water molecule intact gathers enough force to keep it on the surface of water.