Teaching children about the rock cycle can be an enjoyable experience for both children and adults, and it is a great way to get kids interested in the basics of geology. The rock cycle is the process by which igneous rocks are formed, broken down, reformed, and then pushed back down into the core of the earth, from whence they came. Explaining this to children can seem difficult at first, especially if you aren't too sure of the details yourself. Going over the basics can give you a good idea of how to explain it to a child.
Explain igneous rocks to the child. Igneous rocks are made from magma, which resides deep inside the earth. Tell the child about the structure of the earth, if they don't know about it. Eggs are a good metaphor -- the shell is the crust, the white is the mantle and the yolk is the core. Explain that igneous rocks are formed when lava is spewed out onto the earth's surface by volcanoes. The lava eventually cools, and becomes an igneous rock, such as granite, obsidian, or basalt.
Tell the child about erosion, which is the process by which rocks are worn down over time by weathering and the freezing and thawing of water. Find a stone and have the child examine its surface. Generally, the stone will have many small crevices, cracks, or imperfections. Explain that over time, rocks break down as rain beating down on them and water freezes and thaws in the cracks. This is what happens to igneous rocks over time.
Explain how sedimentary rocks are formed. As the bits of igneous rocks are transported over the years, eventually many broken bits of rock, or sediment, will collect at the bottom of a sea or a lake. The constant water pressure compresses the rock pieces, causing them all to squash together and form a sedimentary rock. Animal skeletons are often caught up in this compression process, and preserved as fossils. Limestone, sandstone, and conglomerate are three types of sedimentary rock.
Tell the child about metamorphic rocks. Metamorphic rocks, as their name suggests, are rocks that have changed form over time, usually as the result of heat and pressure. It's possible for both igneous and sedimentary rocks to turn into metamorphic rocks. Over time, they are forced down toward the centre of the earth, being compressed deep in the material that forms the earth's crust. Have the child to squeeze his or her hands together, and explain that the same sort of heat and pressure, but at a much larger scale, creates metamorphic rocks. The pressure comes when the earth's crust moves, pressure so intense that the rocks actually change. Marble, gneiss, and quartzite are three types of metamorphic rock.
Explain how the rocks are eventually pushed so far down into the earth that they are heated by the magma in which they were originally formed. As they are pressed down into the hot centre of the earth, they eventually melt into magma. Now ask the child what magma makes. They will realise that magma was how the cycle began, and thus understand the rock cycle.