I apologise for the belated nature of this post; I’ve been fighting off a migraine all day.

Today’s reaction shows up another of the fundamental rules of chemistry: it is exactly like cooking, only with chemistry you mustn’t lick the spoon.

Interesting compounds are everywhere. The one we’re using today is part of a very useful class known as indicators. “Indicators” on its own is obviously not a very informative name, since it gives no clues to what is being indicated, but it’s become the shorthand for any compound that visibly changes colour when the pH of its environment changes.
The pH is a measure of the concentration of H+. Any molecule that contains hydrogen has a point where it will start losing H+ ions to the environment, depending on the concentration of positive and negative ions that are around. Some coloured molecules have a different colour depending on whether the hydrogen is bonded, or has been lost as H+. So by putting those molecules into a given environment and watching what colour they turn, you can figure out the pH.
Litmus is the classic indicator, giving its name to the “litmus test”, which has made its way into popular culture. But it’s by no means the only indicator out there, or even the best. It’s certainly not the easiest to get hold of.
The easiest indicator to get hold of is found in red cabbage, which is actually a purple colour. If you cut up and boil some red cabbage, and then strain out the juice, or even better blend it first and then strain it, you end up with a solution of a dye molecule which reacts to pH. It will go red or blue depending on what you add to it. Add things like vinegar, baking powder and salt to small portions of your red cabbage juice, and see what colour it goes. Use that information to test other things around your house and deduce whether they’re acidic or alkaline. It’s fun.