I'm joining a rock climbing club that requires an evaluation test in order to become a member. (I think this is a great idea.) They have a detailed handout listing exactly what you need to know for the test, including a bunch of knots. Most of them I was familiar with, but one knot they listed was the slip knot, which I've never used for climbing. Googling turns up this page, which says
You might use it to sling a horn ("chicken head") of rock for example.
What would be the right way to do this? After you tie the knot, pulling on one strand (call it strand 1) tends to tighten the loop, but the other strand (2) won't support tension. Presumably you need to do something else to make it safe. Do you tie a stopper knot in strand 1? Tie the two strands into a butterfly? I'm assuming that in this application you wouldn't have tension on either strand yet, and you may only have access to one end.
Is the slip knot the best knot for this purpose? Are there other rock-climbing applications of the slip knot?