Rosin

The sap from pine tree. It is a sticky substance that is hard to wash of hands.

Rosin is commonly used as a flux in metallurgy, especcially for soldering.

Bows for musical instermunts such as the violin are normally rubbed with rosin to increase the grip on the strings.

It can also be used to demonstrate resonance in aluminium bars by stroking them with rosin tipped fingers.