Fluorescein (Glowing rivers)

Edit: Looks like I might be off on this.

If you're in Chicago, the river will be green this weekend. For that, you can thank fluorescein:

Fluorescein is strongly fluorescent - if it absorbs a photon, 97 times out of 100, it will be re-emitted as light (see yesterday's entry on fluorene for more information).

Fluorescein is a phenomenally useful fluorophore; it finds use in biology to fluorescently tag molecules, as well in medical imaging.

Happy St. Patrick's day!