Can you tie a knot without letting go of the ends of the string? Looks impossible, doesn't it. But you can do it if you know the trick!
Get a piece of string about 2 feet long (can be any kind of string or rope, and any length, but not to short, and not too stiff)

1. Cross your arms, so that one hand is under, touching your side, and the other is over, touching the opposite upper arm.

   
   

    

2. With your arms still crossed, pick up the two ends of the string (this is a bit tricky, but you should be able to do it with some practice).

   
   

    

3. Now, without letting go of the string ends, uncross your arms.

   
   

   

   

    

Presto! As you pull, the knot your arms made will transfer to string.

If it doesn't work, try crossing your arms like in the picture again. They need to really cross, not just have one on top of the other.