try matrix multplication. I mean a real matrices, say 10000x10000. And once you're there, try a LU decomposition of matrix, with proper numerical erro handling. You'll soon get to the point where you fell it's harder than you tought. And BTW, matrix multiplication or LU decomposition are super fundamental stuff when solving equations (which in turn is super fundamental when you want to compute bridges, constructions, aerodynamics, etc...)