For each number you're shown, click Yes if it's a prime number, or No otherwise.
Try to correctly sort as many numbers as possible in a minute.
If clicking on the buttons is too slow, you can press y or n on the keyboard instead.
If you'd rather let the computer decide what's prime and what's not, go to the main Is this prime? website.
Teachers: You can set a cap on the numbers shown by setting the "maximum number" setting. For example, a maximum number of 30 will show you all the odd numbers under 30 first. If you correctly sort all of those, it'll start showing bigger numbers.
You can also change the time limit.
There are some stats. I did an analysis of the stats in May 2016 - there are some odd patterns in the data!