Frequent playing of the piano leaves marks on the keys. Dust and body fat from the fingers accumulates between the keys and also on them.

 

The piano keys can be cleaned thoroughly with a little toothpaste and a toothbrush, even in the spaces between the keys, and then all toothpaste residue can be removed with a damp cloth. Toothpaste as a cleaning agent not only works on ivory keys (which were actually the teeth of an elephant), but also on newer plastic piano keys.