Suppose you have a coffee mug made of stretchy and expandable material. How do you mold it into a donut that has a hole inside?
Take a straight string of finite length. Put the right half on top of the left half to form a hoop, and then pass it (the piece now on top) through the hoop from underneath. Glue the two ends together to form the trefoil knot.
Draw a sequence of pictures to illustrate the formation of the trefoil knot.