On very dry days, when you use a comb or a brush, your hair sometimes stands on the end and maybe even sticks to the comb or brush. explain why this happens in terms of electric charge.

Friction occurs when you rub surface of one object to another which can provoke a electronic charge. 

When you rub a balloon against your clothes and it sticks to the wall, you are adding a surplus of electrons (negative charges) to the surface of the balloon. The wall is now more positively charged than the balloon. As the two come in contact, the balloon will stick because of the rule that opposites attract (positive to negative).