I am going to show you how to tie a very interesting knot that can be used for various sorts of things. It is reliable, but on the other hand, you can untie it very easily and quickly.
Find a stick, branch or anything similar and place the rope as in the picture.
Pull the the loop on the right through the loop on the left.
Tighten a little bit.
Make another loop on the left end of the rope and pull it through the loop obtained in the previous step.
Tighten the knot and you are done. If you pull the right end of the rope, you will tighten the not even more, but if you pull the left one, you will untie the knot.
You can additionally secure the knot by putting a stick in the loop. You will have to remove it when you want to untie the knot.
Step 7: Action
The knot can hold the weights, but if you pull another end of the rope, the weights falls. You can use this knot for various sort of things, for example to tie your dog while you are in the shop.