Three types of rock are formed through different processes. Igneous rocks result from intense fire or heat, sedimentary rocks form from layers of sediment -- bits of rock, sand and decaying plant and animal matter -- and metamorphic rocks were once sedimentary or igneous but changed because of heat and pressure from the movement of the Earth's crust. Show the class several examples of each type, and explain the differences. Give them each a small bag of rocks to sort into the three different types.
Minerals are made of one substance, and rocks are made of minerals. To help your students understand, have each one make a one-inch ball of each of three colors of modeling clay; red, blue and yellow. Each color of clay is pure and does not contain any other color. They represent minerals. Have them mix all three colors of clay balls together. If they mix them long enough, the ball will become a brown/gray color, representing a rock made from several different minerals.
Rocks contain air spaces that can hold water, even if they appear to be solid. Ask your students to bring one or more rocks to class. Place them on a cookie sheet, dry them out in a 150-degree oven for three to four hours, and allow them to cool. Label the rocks with a permanent marker. Help the students weigh their rock and record the weight. Put all the rocks into a container of water, and soak them overnight. Remove the rocks the following day, and dry them with a towel. Reweigh each rock. Help the students calculate the weight difference caused by water in the rock.
Your students think of rocks as being hard. Let them pass around and handle examples of rocks of different hardness, or density. Some rocks crumble easily or break off in layers or sheets. Give each student a piece of real (calcium carbonate) chalk, a common rock and a glass half full of lemon juice or vinegar labeled with their name. Explain the chalk is a soft rock, and show them how to break it in half. Have them put their chalk pieces and the common rock into the glass, and check the glass each day for several days until the chalk dissolves.