Nothing in the pineal gland is ferrous. Magnets, no matter what they are made of, produce a field that acts strongly upon ferrous metals. In short, there's nothing there to stimulate with magnetism.
It's similar to the reason that people familiar with biology and physics cringe whenever Magneto, in the X-Men comics and movies, uses his magnetic powers to attack someone. It won't work, Erik! (Now, if he were to use his powers to, say, pick up an I-beam and smack someone with it...)
Edit: Actually, on reflection, the movies aren't quite so egregious about this - I only recall Lensherr using his powers on Wolverine (whose adamantium skeleton might well be ferrous - the comics aren't clear on the question), and on a prison guard who'd been injected with massive amounts of iron (although one wonders why his body didn't filter some of that out before he went on duty...). The comics still get this wrong with dismaying frequency, however.
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Sodium is a metal that reacts explosively when exposed to water. Chlorine is a gas that'll kill you dead in moments. Together they make my fries taste good.