Surprisals add where probabilities multiply. The surprisal for an event of probability p is defined as s = k ln ⁡ ⁡ ( 1 / p ) . If k is { 1 , 1 / ln ⁡ ⁡ 2 , 1.38 × × 10 − − 23 } then surprisal is in { nats, bits, or J / K } so that, for instance, there are N bits of surprisal for landing all "heads" on a toss of N coins.