Tides

If you've ever been to the beach, I'm sure you've noticed how the water is high up on the shore at some parts of the day and it is very low at other points of the day. These are called high and low tides and they are caused by the moon and the sun and their position in relation to the Earth. Force is inversely proportional to distance squared, so the closer the moon is to a point on the Earth, the stronger the force between them will be. Likewise, with different points of water on the Earth, closer blobs of water to the moon will have a greater force on them than blobs of water on the other side of the Earth. This causes a bulge in the Earth's water; it has an ovular shape which allows for high tides to occur twice a day and low tides to occur twice a day across the globe, happening on opposite ends of the world at the same time. At different times of the year, the position of the moon in relation to the sun causes what are called spring tides (very high high-tides and very low low-tides) or neap tides (tides that are not as great in difference). Although I just gave a very brief summary of this relationship, the video below explains it quite well. Adam Hart-Davis uses food to demonstrate the moon's movement, the Earth's movement, and their effect on the Earth's tides. The visuals are very helpful. He could have included more detail about the process but I think this video gives a very good, simple explanation of tides.