ProjectWET's A Drop in the Bucket* demonstration.
-97 blue beads
-2 white beads
-1 green beads
Students string their beads onto a length of lanyard lacing. They can string them in any order - I happen to prefer the symmetric approach, but the white and green beads can be placed anywhere on the cord.
The beads represent all the water on the planet. The blue beads are salt water. The white beads are frozen water. The green bead is liquid fresh water.
It's a great visual reminder of how precious a resource fresh water is. I've done this activity with students as young as preschool (I count out the 97 beads for them) and as old as 6th grade.
*You start with 1000 ml of water - that's all the water in the world. Pour out 30 ml.
970 ml are salt water. 30 ml are fresh water.
From the 30 ml, pour out 6 ml.
24 ml are ice. 6 ml is liquid fresh water.
From the 6 ml pick up 1 ml.
5 ml are unusable (pollution, etc.). 1 ml is usable water.
So in summary:
970 ml - salt water
24 ml - trapped as ice
5 ml - polluted or otherwise unusable
1 ml - water usable for human consumption