Two well-polished metallic plates very close to each other in a vacuum feel a (slight) mutual attraction. This is the experimentally verified Casimir effect, deeply linked to the fact that the ground state energy of the harmonic oscillator is positive, and to the far more intriguing fact that in a sense and infinity of such oscillators live in the vacuum. Modeling the Casimir effect we have to first confront the great plague of the theory, the occurrence of infinite quantities in the calculations. In this case, though, a simple procedure allows canceling the infinites and getting a meaningful result.