Why is the area under one hump of a sine curve exactly 2?

I was talking with a student recently who told me that he always found the fact that \$latex int_0^{pi} sin x , dx = 2\$ amazing. “How is it that the area under one hump of the sine curve comes out exactly 2?” He asked me if there is an easy way to see that, or is it something you just have to discover by doing the computation.