In addition to what has already been said, I want to point out that there is a normalization step as well. The amplitudes are always calculated in such a way th

In addition to what has already been said, I want to point out that there is a normalization step as well. The amplitudes are always calculated in such a way that the probabilities would never add up to more than 100%. This normalization is generally baked into the wave function of the system you are studying, but it can also be done separately. Incidentally, amplitudes are actually complex numbers. You can think of them as little arrows, like this: →, or this: ↖. In fact, these arrows are also rotating with the passage of time; they trace out little circles. To calculate the probability, we square the absolute value of the complex number. The absolute value of a complex number is equal to the length of the arrow, and squaring a length gives you an area. Thus the probability is essentially the same as the area of the circle traced out by the rotating arrow. Events with high probability correspond to long arrows (big amplitudes), and low probability events have short arrows (small amplitudes). Amplitudes can cancel out when added together if they point in opposite directions. Thus we observe that some sequences of events have very low probability. We sometimes say that these events interfere with each other. Sometimes this interference seems mysterious, as in the double–slit experiment, and other times it seems very mundane. In real life we rarely bother to calculate the probability that the batter will hit the ball before the pitcher throws it, but calculating the correct answers in quantum mechanics requires taking into account many such unusual events.