Read the following paragraph and answer the question. Give ONLY the answer, no other words or explanation.

For example:

Answer: <most likely answer, as short as possible>.

Paragraph: 
Electrons are transferred from hair to a balloon rubbed against the hair. Then the oppositely charged hair and balloon attract each other.


Question: When a balloon is rubbed against hair, why do they attract each other?
opposite electron charge
Because the balloon and hair become oppositely charged.