Metamorphic Rocks change from one form to another through Earth's heat and pressure. To demonstrate how heat and pressure can make substances change form, it is fun to make these bar cookies.
First put 1/4 cup melted butter in a 13 x 9 inch glass baking dish. Sprinkle over it 1 1/2 cups graham cracker crumbs (you could use cookie crumbs instead). Pour 6 ounces of sweetened condensed milk over this. Sprinkle on, in layers, 3 1/2 ounces of raisins or coconut, (We used a combination.) 4 ounces of your favorite nuts or seeds (whatever you happen to have on hand.), 4 ounces of chocolate or butterscotch chips. Top with 3 1/2 ounces granola.
This represents the pressure exerted on metamorphic rocks.
Take off the waxed paper and bake about 25 minutes in a 350 degree oven. Cool. Slice into bars. Notice how some of the ingredients have changed a lot while others have made more subtle changes. This is also true for metamorphic rocks.
I have also seen the idea of making Rice Krispie bars instead. Same idea.
I don't remember where I got this idea, so I cannot give credit.