FACT: Teenagers are unable to hang towels up in the bathroom.

SOLUTION: 3D print yourself this handy simple towel clip, 
insert a 20mm diameter ball, the clip has enough 'give' to allow the ball to push in from the side - but it won't fall out (the ball can be bought or 3D printed as well).

Slip the towel up behind the floating ball - ball drops down locking the towel firmly in place. A quick pull on the towel to release it.

Fix to the bathroom door or wall and never again deal with a towel covered bathroom floor!

Print at 0.2mm, high% infill - no support or raft needed for clip.
Ball may require some support if you wish.
Print ball at 100% infill to give it some 'weight' or purchase a 20mm steel ball or marble from any good toy / hardware store.

Scale the Towel clip up or down for other sizes of ball insert.

Enjoy!