A magnetic compass in a test tube sounds like a magic trick, but it's pure physics. In a solution of fumarate (a salt of malic acid), carbon-13 nuclei behave like tiny spinning tops.
In a small Petri dish sits a droplet of liquid the size of a pinhead. An ordinary scanner wouldn't even notice it: the signal is too weak. But bring a tiny magnetic field sensor close to the droplet, and the picture changes.