Step One: Open Safari.
Step Two: Open Activity monitor.
Step three: Rig it so you can see both windows at the same time (or use the floating CPU window)
Step four: Open a big slashdot discussion. Apple's iPod release let's say.
Step Five: Have the new comment system on slashdot.
Step six: Scroll.
Watch as your CPU usage goes out of this world.
Bonus: Install a temperature monitor, and watch as you scroll and your temperature spikes too.
It's too bad last time I tried Firefox2 on OSX I tried to shoot myself.