As kids, we made toys from anything we could lay our hands on. Just needed to put on our thinking caps and get creative juices flowing.
A bottle cap was fair game material. After flattening with a hammer, we punched two holes in it with a big nail. Then a coarse string was threaded through those two holes, and the free ends then tied together.
Once ready, the string would be stretched between two hands to form an elongated loop and then the flattened cap would be whipped around until there was a good twist in the stringed loop. Upon repeated pulling and releasing rhythmically, we got a spinning cap that oscillated back and forth. That was fun.
Caution : if the string burst, the spinning cap could fly off like a circular saw blade.