The probability that it rains in London on any particular day is 0.4.
If it rains then the probability of it raining the following day decreases to 0.15.
Consider the next two days.
What is the probability it doesn't rain on either day?
.6 * .6 = .36
the .15 does not enter because it did not rain the first day.
Yikes, it rains 40 % of days there ?
and we complain about droughts.