The probability of a sure event is 1.
A sure event is something that is guaranteed to happen. For example, if you toss a fair coin, the probability that it will land on either heads or tails is certain, which means the probability of getting either outcome is 1 (or 100%).
In probability theory, any event that is guaranteed to occur during an experiment has a probability value of 1. Conversely, the probability of an impossible event (an event that cannot happen) is 0. This forms the basic foundation of probability, where the probability of any event ranges from 0 to 1, with 0 indicating impossibility and 1 indicating certainty.