Angle. Comparing angles
Two rays with a common origin together form an angle: the rays are called its sides and the common starting point its vertex. In the notation ∠AOB the vertex O is written in the middle. If the two sides of an angle together make up one line, the angle is called a straight angle. An angle that is not straight divides the plane into two parts: the part between the sides is the interior, the rest is the exterior. Axiom: a non-straight angle can be laid on a given ray into a given half-plane in exactly one way. To compare angles we lay both from one ray into one half-plane: if the sides coincide the angles are equal; if one lies inside the other, it is smaller.
“Paper angles”: cut two angles from paper (with an adult’s help) and lay one over the other to see which is bigger.