Mathematics
Monte Carlo Methods for Numerical Integration and Simulation
Quick fact
Monte Carlo integration has an error that decreases as 1/√N, independent of the dimension of the integral. This makes it the only practical choice for high-dimensional integrals (e.g., dimension 10), where deterministic methods become exponentially costly.
Why this is interesting
You have a complex integral you can't solve. Instead of using a systematic grid, what if you just threw darts at it blindly? That's the surprising idea behind Monte Carlo methods.