Cross the planks to reach the goal
	Fall into the water
	If there's not equal weight on each side the planks will tip

There is a row of ten seesaw-like planks across a lake. The pairs must cross the water by stepping from one seesaw to another - standing only on opposite ends of each plank.
If weight is put on just one side of a plank it will tip up; so stepping onto each plank unison is the key to making it across.