High tide is when the ocean's water level reaches its highest point along the shore. Twice each day, the moon's gravity pulls on Earth's oceans, causing the water to rise up the beach, cover rocks that were dry hours before, and sometimes reach all the way to seawalls or dunes.

If you visit the beach in the morning and mark where the waves lap against the sand, you might return six hours later to find the water has climbed much higher up the shore. That's high tide. Then, about six hours after that, the water will retreat back down the beach to low tide, exposing tidal pools, sandbars, and areas where you can hunt for shells and sea creatures.

Sailors and fishermen pay close attention to high tide because boats can enter harbors more easily when the water is deeper, and some fish feed more actively when the tide is high. People who live near the ocean learn the tide schedule because what was a dry walking path at low tide might be underwater at high tide. The difference between high and low tide can be just a few feet in some places, but in the Bay of Fundy in Canada, it can be over fifty feet.