I have an exhibit at the Vancouver Mini Maker Faire this weekend (June 6-7, 2015). It's a DIY Spectrophotometer, that is very obviously home-made, but allows you to see the absorption spectrum of different liquid samples. For example, if you look at the spectrum, you can see for yourself that rubbing alcohol and vodka are very very different substances. (Don't drink rubbing alcohol!)
You can see a gallery of spectra here, or come try it for yourself at the Faire.