diamagnetism
/ DY-uh-mag-net-iz-um /
Bring a magnet near almost anything — water, wood, a slice of apple, your own finger — and it gives a barely-there shove the other way. That faint reluctance, the tendency of ordinary matter to push away from a magnetic field rather than toward it, is diamagnetism. It is so weak you would never notice it without sensitive instruments.
Here is the mechanism. When a magnetic field switches on, it nudges the orbiting electrons inside every atom, and they respond by adjusting their motion in just the way that opposes the change. This creates a tiny induced moment pointing against the field. The effect is present in absolutely all materials, because every atom has orbiting electrons; it simply gets buried whenever the atom also has a stronger leftover moment of its own.
Diamagnetism matters because it is the one truly universal magnetic response — the quiet baseline underneath everything else. The common misconception is that 'non-magnetic' materials have no magnetic behavior at all. In fact they are weakly diamagnetic, and with a strong enough field you can use this to levitate water droplets, or even a live frog — harmlessly, by floating it on its own faint push-away.
In 1997, physicists floated a living frog in midair using a powerful magnet. The frog is mostly water, water is diamagnetic, and a strong enough field pushes every water molecule away just hard enough to balance gravity.
Diamagnetic levitation: the frog's own water is gently pushed away by the field, holding it aloft.
Diamagnetism is the only magnetic response that points against the field rather than with it. A superconductor is, in effect, a perfect diamagnet — it expels a field completely — but that comes from a different, much stronger mechanism than the feeble ordinary version described here.