Simplify the following text for a Grade 5 native English-speaking person.
Original: Euclid uses right angles in definitions 11 and 12 to define acute angles ( those smaller than a right angle ) and obtuse angles ( those greater than a right angle ) . Two angles are called complementary if their sum is a right angle .
Simplified: When two lines cross each other so that all the angles have the same size , the result is four right angles .