At first, the tea bowl is empty, but when you slowly pour water into it, you will find a silver coin in it. What happened?

There are coins in the teapot, which fell into the tea bowl when pouring water.

Either your problem is wrong, there must be coins in the tea bowl at first, but it is a matter of perspective.

Let me correct it for you: put a coin at the bottom of the cup, move the cup to a place where you can't see the coin, and pour water into the cup to see the coin.