So let's talk potassium cyanide. Terribly deadly, a spy's best friend if captured. Kills you pretty quick-like in a small amount.

Throw an iron atom at it, now it's potassium ferricyanide. Instead of a white powder, now you have beautiful dark red crystals that are largely non toxic (don't put it in strong acid, though). The molecule itself looks like a spiky ball of CNs protruding from an Fe³ ion, with some potassium for flair.

Ok, let's get wacky and throw another potassium ion at it. All of those CNs straighten out and line up, it turns yellow, and now it looks like a kind of sword. Now let's put it in a human GI tract.

Once there, that extra potassium ion looks over at some receptors on smooth muscle neurons and thinks it would rather be there.

So, it pops off of the salt and slides into that neuron, while the salt turns red again and retracts into a spiky ball in protest.

Meanwhile, that potassium is triggering increased smooth muscle motility, which translates to: you'll be pooping a lot, and sometimes doctors do it on purpose.