You have to build atleast a chain of three sheeps to remove these from the gameboard.
If you manage to remove 8+ sheeps, falling sheeps included then this will generate a new line of sheeps on your opponents gameboard.

Defeat your opponent by bumping as much extra lines on the opponent's gameboard as you can.

Avoid crossing the lower borderline yourself.

[id]http://flashgamedistribution.com/swfs/sheeplets.swf[/id]