I cannot take credit for this (though I can't find the original source to cite it either -- sorry to the author); but, this was shared by a chemistry professor I had back at my University:

Roses are red,

and if violets look blue,

that's because anthocyanidins

can vary in hue.

All the chemistry Geeks (like me) smirk if you say this one; but, everyone else fully and rightly questions your sanity. ^_^