3.A: Particles and waves
Besides the uncertainty principle (described in the previous section), there is another feature of quantum mechanics that you will find quite different from your everyday experience. According to quantum mechanics, things that we are used to thinking of as particles are actually described by waves. Conversely, things that we are used to thinking of as waves, like electromagnetic radiation, are also particles. The matter waves studied in quantum mechanics are called wave functions (often written using a Greek ``Psi''.)
In quantum mechanics the probability of finding a particle at a certain position at time t is the square of the wave function. Since the probability is the square of a wave function, the probability is guaranteed to be a positive number, as a probability must always be.