Mr. Ochoa: Central Texas is full of caves, and some of them are big enough to walk through. Today I want you to understand how a cave can form inside solid rock. The answer is water, and a lot of time.

Mr. Ochoa: Much of the Hill Country sits on a kind of rock called limestone. When rain falls, it picks up a gas called carbon dioxide from the air and the soil. That turns the rainwater into a very weak acid. It's much too weak to hurt your skin, but limestone slowly dissolves in it.

Mr. Ochoa: The water seeps into tiny cracks in the limestone. Over thousands and thousands of years, it dissolves the rock little by little, and the cracks grow into tunnels and rooms.

Mr. Ochoa: Once a cave fills with air, something else happens. Water dripping from the ceiling leaves behind tiny bits of mineral. Drip by drip, those bits build up into stone icicles called stalactites. Where the drops land on the floor, mounds called stalagmites grow upward. Here's how to remember which is which. Stalactites hold tight to the ceiling. Stalagmites might reach them someday.