Algodoo

You may have heard that the Moon causes tides. But why? It's because the Moon's gravitational field isn't uniform, it gets strongere the closer you are to it. Here I show some water deforming due to a non uniform gravitational field. The drops that are closer will experience stronger gravity, and the ones behind kind of lag, due to a weaker gravitational field, making two tidal bulges one behind and one in front.