Probably the most easy bike tool to check the wear of your bicycle chain.

The Rohloff Caliber 2 is a precision made chain wear indicator. It allows the measure of chain wear in seconds and indicates if the chain is too worn for steel or aluminum sprockets/chainrings. After extended use, a chain will no longer fit correctly in the teeth of the sprockets, due to elongation and wear. If the chain is not replaced, then chainrings and sprockets may be damaged, and eventually, the chain will skip over the sprockets. The Caliber 2 allows you to quickly determine chain wear before your chain gets to this condition.