It's all about gravity. Gravity is a strange force of attraction that pulls two (or more) things together.

The earth's gravity pulls us down and the muscles of our legs hold us up. If you jump up the earth's gravity pulls you down again.

As the moon passes over the sea, 1/4 of a million miles away, it pulls the sea up a bit. Hence a high tide. Will that do for a start?

L in C#