Watercolor painting, unlike most other forms of painting, is all about preserving the white of the paper. There is no such thing as "white" watercolor; there are similar, other water-based media, such as gouche (pronounced "gwash") that contain white pigment, but when looking at most "pure" watercolor paintings, any bits of white are simply the underlying layer of paper.
For much of watercolor's history, then, this meant artists had to go to great lengths to avoid painting over areas that were to remain white. Wax resist, scraping with razor blades, and careful brushwork were the accepted norm. One moment of inattention could ruin untold hours of work. Then, in the 1970s several products started showing up on the market that would make masking much easier. Known as "liquid-gum", "liquid frisket" or simply, "miskit", they were a form of latex polymer that could be painted onto paper, then pulled off; much like wax resist, but without the unwanted sticky residue. Initially many traditionalists snubbed these newfangled products, but by the late 1980's miskit had become widely accepted as yet another tool in the watercolor genre.
I myself have found it particularly useful when painting subjects with dark, sweeping washes behind them, or for reserving the pristine whites of eye highlights or individual stars or snowflakes. I've found that like any technique, it can be used too much, and if done incorrectly, miskit can make a painting look a little too flat or cut-out; so I try to only use it if I feel I can't get a particular effect any other way, or if I'm overly concerned that I'll paint over a delicate area.
Seeing as my misket use can be infrequent, then, I usually buy a new bottle of the milky-white substance about once a year or so, even if I haven't finished the old bottle. Usually the stuff just dries up or gets a strange rubbery consistency that makes it difficult to work with. (I've heard you can add a bit of ammonia to make an old bottle go further, but haven't tried it yet.) So imagine my surprise when I recently opened up a bottle of unknown vintage to find.....Yeaach! It had not only gone bad, but somehow begun to resemble a cultured petrie dish. I've encountered curdled milk and moldy bread, but this was the first time I've come across spoiled miskit. Nasty, nasty stuff.
They really need to put "BEST IF USED BY" dates on it.