It's like raining on the roof of your mouth," says a middle-aged person who tried it.

"Better than Rice Krispies," says one of the younger set.

Reactions from some hard-to-please older people range from "ouch" to "disgusting."

"It" is Pop Rocks, a new carbonated hard candy that sizzles, snaps, pops and tingles in your mouth, and brings grimaces, faint similes and started reactions from first-time users. "Explore the far reaches of your mind," says the package.

Since it has no apparent purpose or social significant - two key fad criteria - Pop Rocks promise to win a place in the npatheon of freaky gimmicks that infect America from time to time.

All of which brings more smiles than grimaces to General Foods, maker of Pop Rocks, which has sold "hundreds of millions" of the packets, according to a spokesman, without a word of advertising.

In fact, in true fad fashion Pop Rocks seem to be something of a hot underground item. They're being bootlegged to New York and sold on the streets and in a few stores for three and four times the normal price.

Rumors about their existence and even a new samples of the 20-cent package have reached Washington, but exactly when the product will be on sale here, the manufacturer isn't saying.

Meanwhile, reports like a recent one in Advertising Age (confirmed by General Foods) that an overheated load of Pop Rocks blew the doors open on a delivery truck have raised questions about the product's safety. Speculation that Pop Rocks might harm the esophogus or the taste buds led NBC consumer reporter Betty Furness to take to the air recently to calm parent's fears.

The secret ingredient of General Foods' smash seller is carbon dioxide, about one-tenth the amount that's put into soft drinks to make them fizz. While a Pop Rocks left on the tip of the tongue with one's mouth open will "explode," ingredients forthe product were okayed by the Food and Drug Administration before General Foods started to test market it in 1976.

Meanwhile, the company's hot product has its own unique sales difficulties.

The company says Pop Rocks can't be sold in areas where the average temperature is over 85 degrees. And sales are being suspended over the summer for the same reason. The shelf-life is also limited because the carbonation dissipates over a period of time and then you're left with "just another hard candy."

But wherever they appear, says the company, they are being hoarded. It's not unusual to see people walk out of a store, carrying a large shopping bag filled with nothing but Pop Rocks or Star Dust (the former are large rocklike pieces; the latter, dust-like bits).

Which explains why the candy is outselling most, if not all major candy bars "in any market where they are introduced," General Foods says.

Moreover Alka Seltzer.