Hey, so the sum of all three angles in a triangle is 180 (in degrees). So when 2 lines meet there is an angle between them, this is going to be a number between 0 and 360. 1000s of years ago when they decided how many degrees should be around a point they thought 360 would be a good number. Now an important rule with shapes is that the sum of the angles inside is always the same, so long as you keep the number of side the shape has the same, it doesn't matter how you stretch it or pull it. So imagine a straight line, which we will call a triangle (you have to use your imagination because a straight line isn't a triangle). But to do this the 2 ends of the line will be 2 of the corners of the triangle. And they will both be 0(degrees) because there is no space between a line and itself. So we have 0+0 as 2 of them, but for it to be a triangle there must be a third corner, which isn't really a corner at all, but again use your imagination. This corner will be somewhere between the to ends, and it will be flat, no bend in it at all. Now we know that around a point there is 360 degrees, so around half a point, or on this straight line, there must be 360/2. Or 180. So 0+0+180=180, which will stay the same even if you stretch the triangle out so it is no longer a flat line