The Poisson approximation to the binomial

The binomial distribution B(n, p) may be approximated by a Poisson distribution with parameter $\lambda$, where $\lambda$ = np, provided that n is large and p is small. Investigate how good the approximation is for different values of n and p. You can zoom in or out using the toolbar.

A rule of thumb is that p should be less than 0.1. What do you think? How large does n need to be to get a good approximation?