In the prison of the magical world Azkaban all prisoners are kept in solitary cells and have no way of communicating.
One day Reginald Candlenut, an auror, burst into the office of chief warden Teophilius Betelfax.
"Mr Betelfax, I fear that two prisoners have somehow found a way to communicate despite the strong measures. Thanks to the charm Amplifo Petri I heard crackling in the prison walls. I made a large number of measurements in equal intervals and found that in 45% of them there is silence, in 38% there is a single crackling sound and in the remaining 17% there are two crackling so
