I think it's a good idea to tell about block generation in Nxt.

Alice has 2500 nxts on her account.  Last time she found a block 4 days ago.  Her money is like a mining rig with hashpower equal to 2500 * 4 = 10000 GH/s.
Bob has 1000 ntx on his account.  He was on vacation and hasn't opened his account for 20 days.  His money is like a mining rig with 1000 * 20 = 20000 GH/s hashpower.
Each time Alice or Bob opens their account they have a chance to generate a block.  More money they have, higher a chance to do it.  More time passed since the previous generated block, higher a chance to generate a new one.
Even a small amount can generate a block by accident.
An exchange that holds coins of its customers can afford to provide the service with 0% fee.  It will still earn money via block generation.