In knitting, a slip knot is often used at the beginning of a cast on.
How to make a slip knot
Hold the yarn in your left hand so the tail hangs toward you. With your right hand reach behind the yarn to grasp the yarn tail and pull it to the left. Poke your left thumb and forefinger through the loop created in 2) and grasp the yarn tail with your thumb and forefinger. Hold the yarn and tail with the right hand and pull the yarn tail backwards through the loop. Put the loop onto the needle or hook and pull the yarn tail to loosely tighten the knot.