Find two buttons that you think are part of the same group, then click both of them to find out.
Keep going until there are no more buttons left to merge.
If you get stuck, ask one or more friends for help; this puzzle is much more fun to solve collectively.
It is also vastly harder to solve on small-screened devices like phones.
The page does save your progress, so it is okay to close your browser or turn off your computer and come back to it later.
make_groups.html is by Thomas Colthurst and is released under a Creative Commons Share Alike with Attribution License. Many thanks to my playtesters: Rachel, Arun, Jeremy, Alison, Billy, Michael, Sally and Jon.