Why is the website sometimes not available?

Note that you must be registered to access all of the simulations. Unregistered users can access the more basic simulations and all of the general website content. Since the simulations can be computationally intensive, it makes sense for us to limit the number of simultaneous users. The number of users currenly online is always shown in the right sidebar. Currently, we have a limit of 200 anonymous users and 75 registered users -- for a total of 275 users at any time. If the website is slow or appears to not be available, then please try again later. The user limits may be reduced in the future, if the load on the system is too high. If the website becomes very popular, we may consider switching to a dedicated server (provided we have the $$$, of course :).