The gravity of the moon and sun pulls on the Earth. The gravity is strong enough to affect the oceans. It causes the seas to rise and fall. We call this the tide. As the Earth rotates we get a high and low tides twice each day. The moon moves around the Earth every 28 days, so the timing of the high and low tide changes every day during each month.
The area along our coasts between the high tide and the low tide is called the intertidal zone.
Some of our coast is covered in rocks. Often the waves are strong enough to wash away most of the sand from the rocks. At low tide all the hollows and crevices among the rocks can contain water. The waves no longer stir the water in these pools so it is easy to see under the surface.
Click here to see a time-lapse video of the tidal movement over 12 hours.