We will soon study the central limit theorem, which states that a Gaussian probability distribution arises when describing an overall variable that is a sum of a large number of random variables, no small number of which dominate the fluctuations of the overall variable. To study the combinatorics involved in an example where this theorem applies, we will need to work with the factorials of large numbers. Stirling's approximation is an approximation for n! for large n. In this video, we motivate this approximation by comparing the expression for ln(n!) with an integral of the natural log function.