  Bell's theorem is a fundamental result in quantum mechanics: it discriminates
between quantum mechanics and all theories where probabilities in measurement
results arise from the ignorance of pre-existing local properties. We give an
extremely simple proof of Bell's inequality: a single figure suffices. This
simplicity may be useful in the unending debate of what exactly the Bell
inequality means, since the hypothesis at the basis of the proof become
extremely transparent. It is also a useful didactic tool, as the Bell
inequality can be explained in a single intuitive lecture.
