Mrs. Birtwell taught us about "plate tectonics". This means that the continents and ocean floor have moved around for millions of years.
She showed us how a "midocean ridge" in the Atlantic Ocean allows molten rock to come up and harden to make new sea floor. When it does, it pushes the rock that was there before out a little further -- and this is what makes the landmasses move!
She modeled it with a box as the midocean ridge and sheets of paper as the magma, which made it a little easier to understand.