Two to Four Styrofoam Balls (one for each team, found at your local craft store)
Gray Spray Paint
Two to Four Boxes or Buckets (larger than the balls, one for each team)
Spray paint each Styrofoam ball gray to make boulders.
You will need to paint one boulder for each team that's playing.
Let them dry for at least 30 minutes.
At the Party:
Divide your guests into teams of at least 4 people each.
Line up the teams, and have the children space themselves about 6 feet apart from their teammates.
Place a boulder in front of the first player in line on each team, and place a box or bucket 6 feet behind the last player on each team.
Explain to your guests that they must roll their team's boulder to the next player in line, moving only on their knees, and that they are not allowed to pick the boulder up with their hands at any time.
For example, they may crawl and push the ball with their head, or walk on their knees and use their hands to push the boulder ahead of them like a bulldozer.
The last player in line must successfully roll the boulder into the box or bucket.