If you are that mouse I except you wouldn't want to face one of these opposite you on a see saw. You would want someone opposite you of the same size to make things equal and a bit easier for us.
There are lots of things in maths that are equal, which makes things easier for us.
When two lines intersect (cross over) each other, four angles are formed.
The pair of angles which lie opposite each other at the point of intersection are called vertically opposite angles.
Vertically opposite angles are equal as shown in the diagram below.
As an added extra, can you see that the angles on the straight lines add up 180° another mathematical fact.
Let's see how you get on