Piezoelectric crystals are used in

$MnO$ is $:$

Explain the cause of magnetism in a substance.

What type of magnetism is exhibited by $MnO$?

An example of a network solid is

Soldiers of Napoleon's army while at the Alps during a freezing winter suffered a serious problem regarding the tin buttons of their uniforms. White metallic tin buttons got converted to grey powder. This transformation is related to