Angle

An angle is formed when two straight lines meet. The spot they meet at is called the vertex.
Angles are measured in degrees. A few examples will show you how to measure angles:
The sides of a square meet at 90 degrees:

(The little square means 90 degrees)

Here, the two lines meet at 30 degrees:

Here's an obtuse angle of 155 degrees:

Now, here's a problem for you:
What angle do these lines meet at?

Hint: It's half of the 90 degree angle!