You need: a ball, an inflatable globe or beach-ball globe if you have one (or a world map on the floor and a coin or button), paper and a pencil.
If you have a ball globe: Toss it to a partner 20 times. Each time, check where the partner's right thumb lands. Make a tally: land or water.
If you have a map: Stand up, close your eyes and drop a button on the map 20 times. Tally land or water each time. (If it lands on Antarctica, that counts as land!)
Count your tallies. Which got more? Most people get more water, because most of Earth is water.
Then: Make the seven continents out of play dough or paper, put them in the right places on a blue sheet or towel, and label the five oceans with sticky notes.