Mathematics
The Chinese Remainder Theorem and Its Computational Uses
Quick fact
The Chinese Remainder Theorem allows you to solve a system of congruences with pairwise coprime moduli efficiently, and it is used in RSA decryption to speed up the process by a factor of about 4—turning a single large-modulus exponentiation into several smaller ones.
Why this is interesting
Imagine three different clocks showing different times; can you find a time that satisfies all three simultaneously? The Chinese Remainder Theorem says yes, and it does much more—it makes large computations faster and powers modern cryptography.