Create two teams for this activity. Have two 5-gallon buckets and 10 empty 1-gallon milk jugs. Fill each of the milk jugs with water. Designate one person on each team to brush his teeth. Line the children up and give them each a milk jug filled with water. Stand the child who is brushing his teeth over the empty bucket and tell the children to begin. Have a each child with a jug pour the water out at the speed of a running faucet. If the jug is too heavy, aid the child by helping him pour. When both children finish brushing their teeth, check the buckets to see how much water they've used. The team who uses less, wins.
Teach children the importance of reducing waste as well as conserving water by giving each child a reusable water bottle. Tell each child that they can only refill their bottle if they drink all the water in it. This means that children will throw away less water and conserve more of it. To make the water bottles even more enticing, give them non-toxic markers during class and have them draw on or initial the bottle. If there are days where plastic water bottles are use, instruct the children to put their plastic bottles in the correct recycling bin.
Look ahead into the forecast. If the weather calls for rain, place empty buckets outside the classroom window to catch the water. Use this to water the classroom plants. Conserve the rain water and have a different child each day water the plant using the rain water.
After teaching and explaining to each child why water conservation is important, test their knowledge. Give each child drawing paper and art supplies and ask them to draw two or three things that would happen if the world were to run out of water. Once the children are done, have them explain their drawing one at a time.