Tumble dryer balls

Yes very effective. My home science experiment I washed a load of towels and dried them for 1 hour on hot heat - all dried. Following week, same load of towels this time with 2 dryer balls added. I was checking at 20 mins intervals and the towels were bone dry at 40mins same heat setting.

What do dryer balls do?

I've never heard of dryer balls, so I'm intrigued. I'd like to know more.