Stopper knots are an important part of climbing, here are a few of my favorites and an easy way to remember them and tie them using a left underhand loop.
I once saw a friend almost descend off the end of the climbing and there was no stopper knot.
It is my goal to ALWAYS have a stopper knot on my climbing line, whether it is flaked out at the base of the tree or in the bottom of my rope bag.