Dennis Fairman, age 12, of Des Moines, Iowa, for his question:

What are tidal pools?

When you visit an ocean beach, you can watch the tide wash up over the sand, then ebb back to the sea. But not all the shores have wide flat stretches of sandy beach. Along some coasts, towering cliffs come right down to the water's edge. In other places, there are great chunks of tumbled rock among the sandy stretches. This is where we are likely to find tidal pools.

When the tide comes in, it washes over the coastal boulders and floods the sandy pockets between them. As the water ebbs back to sea, tidal pools may be left among the rocks and at low tide the whole region may be left high and dry. The tidal pools fill up and empty twice every 24 hours. Yet countless marine creatures are adapted to live with these changing rhythms    and the teeming life in a tidal pool is a small, wondrous ecosystem.