A particle makes a step of 2 mm to the left or to the right every 0.1 s.

A particle makes a step of 2 mm to the left or to the right every 0.1 s. For one dimension, the mean squared distance is $\langle x^2 \rangle = 2Dt$, where $D$ is the diffusion coefficient. Simulate 4000 particles for 200 steps, fit a straight line to the mean squared distance against the time, and find $D$. Compare with the theory: $D = \ell^2 / (2\tau)$ for steps of length $\ell$ every $\tau$.