Open the arms, make right, acute and obtuse angles, read a protractor and find the missing angle.
Start here. An angle is made when two lines meet at a point. The lines are the arms and the point is the vertex. The size of an angle is how wide the arms are opened. Long arms do not make a bigger angle.
The square corner. The corner of a book or a window is a right angle. It measures 90°. An angle smaller than a right angle is acute. An angle bigger than a right angle, but smaller than a straight line, is obtuse. A straight line is 180°. Anything bigger than 180° is reflex.
Turns. A quarter turn is 90°. A half turn is 180°. Three quarters of a turn is 270°, and a full turn all the way round is 360°.
Using a protractor. Put the centre on the vertex and line one arm up with the 0 on the scale. Then read the number where the other arm crosses the same scale. In this game only the scale that starts at 0 is shown, so you cannot read the wrong one.
Angle facts. Angles on a straight line add up to 180°. Angles around a point add up to 360°. The three angles inside a triangle add up to 180°. When two straight lines cross, the angles opposite each other are equal.
Try this above. In Play, drag the ray to make a right angle, then keep going until the arms make a straight line. Turn on the protractor and read the number. At Challenge me, go all the way round to 360°.
An angle is the amount of turn between two lines that meet at a point. We measure it in degrees. A small angle is like a slightly open book. A big angle is like a book opened flat.
The number 360 can be divided evenly by many numbers (2, 3, 4, 5, 6, 8, 9, 10, 12 and more), so it is easy to split a full turn into equal parts. The idea goes back to the ancient Babylonians.
A protractor is a tool shaped like a half circle with a scale from 0 to 180. You use it to measure or draw angles.
Pick the level, not the age. "Just starting" sorts angles into acute, right and obtuse. "I know the basics" reads a protractor and uses turns. "Challenge me" finds missing angles on lines, around a point and in triangles.
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