Bowline
The Bowline is one of the oldest and most useful knots. The bowline is a very secure knot which will not slip, regardless of the load applied. It is commonly used by mountain climbers to tie their climbing ropes around their waists. It is a very important knot for scouts. It can be used to pull a person out of a ravine.
You make a bowline by forming a loop, or "rabbit hole". The rabbit (bitter or free end of the rope) comes up through the hole, around the tree (standing part of the rope) and back down the hole.