Sizzle

"Caustic", of course, means "corrosive", which is to say "having the property of dissolving by chemical action". Or, metaphorically, "incisively cruel", as in a caustic remark or caustic wit.

That up there is a bag of lye, aka caustic soda. I'd stay well away from it if I were you.

But these are also caustics in a body of water (well, it's a computer-generated image, but that is what caustics do look like, as anyone who has ever been in a swimming pool knows).

and that bright area on the table is also a caustic. (Also a computer-generated image, but still also a good mimic of the real world.)

So a caustic is a curved geometric shape created by the warping of light, and the only possible response is, Yes, but what has that to do with lye?

I'm about to tell you. Of course!

"Caustic" comes from Greek "kaiein", "to burn", by way of "kaustos" and "kaustikos", "combustible, able to be burned". A caustic made of light can focus that light in such a way that it causes something to catch fire--think of a parabolic mirror. And that's it.