Buffy the Vampire Slayer, Season 7

Xander:
Figuring out how to control your magic seems a lot like hammering a nail. At the end of the hammer, you have the power, but no control. It takes, like, two strokes to hit the nail in, or you could hit your thumb.

Willow:
Ouch.

Xander:
So you choke up. Control, but no power. It could take like ten strokes to knock the nail in. Power, control. It's a trade off.

Willow:
That's actually not a bad analogy. Except, I'm less worried about hitting my thumb, and more worried about going all black-eyed baddy and bewitching that hammer into cracking my friends' skulls open like coconuts.

Xander:
Right. Ouch.