my roommate told me a cool puzzle:

there are a large number of soldiers standing in a line,
so many that we don't know how many there are,
and we can't just count with our eyes,

and we want them to all fire their guns simultaneously,
and there's a drum that they can all hear and count the beats of,

but we can only talk to the soldier on one end,
and each soldier can only talk to the soldiers next to them,
so what's the most efficient way to get them all to fire at the same time?

he didn't tell me the solution.. though I'm sure it's online.. but I love having a puzzle :)