What type of elements form covalent bonds?

The diagram shows the positions of the sun, moon and earth during spring tides, when the high tides are at their highest and low tides at their lowest. what is it about these positions that causes these high and low tides?

Frictional forces acting on an object are often converted into energy, which causes the temperature of the object to rise slightly.

Why is the melting of ice a physical change ?