Here's something to remember as you study organic chemistry: Life is built on chiral building blocks — that is, the molecules that make up living things have a particular handedness (designated as R or S), and nature only uses one handedness and not the other.
In Alice in Wonderland, Alice is transported through the looking glass into a world that is the mirror image of our world. Alice, being very clever, speculates that the mirror-image milk in Wonderland might not be good for the cat to drink. A purely scientific question arises: Is Alice right to be worried about the milk in Wonderland?
You bet! If Alice went through the looking glass, all the molecules in Wonderland would be the mirror image of those in the natural world, and the handedness of the molecules would thus be switched from R to S (or S to R). Objects would look similar, but this mirror-image inversion would lead to some chemical problems.
For example, lemons would smell like oranges and oranges would smell like lemons, because R-limonene (a 15-carbon oil) gives the odor to oranges and S-limonene, its mirror image, gives the odor to lemons. In Wonderland, spearmint chewing gum, which gets its minty flavor from R-carvone, would taste like caraway seeds, which get their flavor from the mirror-image molecule, S-carvone. In general, the smells of things in Wonderland would be strange and weird, because many of the odorants in the natural world are chiral, and their enantiomers (mirror images) would likely smell completely different.
What if Alice wanted something to eat? She would be in trouble, because our digestive system only plays nicely with molecules of a single handedness. For instance, we only digest D-sugars and not their mirror-image enantiomers, L-sugars. Thus, any food containing sugars or starches would be indigestible. Additionally, humans only digest L-amino acids and proteins, so meat and other protein sources (like nuts and legumes) would be similarly indigestible. Fat might be digestible in Wonderland, because many triglycerides, or fat molecules, are achiral — the chiral fats might still be digestible if the chirality were switched.
Would the milk be good for the cat, or for Alice? Unfortunately, no, because the milk sugars (largely lactose) would be indigestible, as would the milk proteins. What if Alice had a headache? If she found bottles of acetaminophen (Tylenol), naproxen (Aleve), and ibuprofen (Advil), which pills should she take? Because enantiomers of drugs can be inactive or toxic, and because ibuprofen and naproxen are chiral while acetaminophen is not, she would be wise to take the acetaminophen — only this drug would be identical to the regular drug in the mirror-image world.
Some things would stay the same, however. Alice might be relieved to know that both ethanol and water are achiral, so perhaps she would be wise to take a few Tylenol, head on over to the local pub to enjoy a refreshing beverage, and do her best to stay out of the way of the red queen.