If you were an alien creature who came to Earth to investigate its beings and if, to blend in better, you assumed human form, then you could do a lot worse than Kristen Johnston’s.

Not that Johnston herself blends in so great among the earthling population, come to think of it. She’s 6 feet tall with legs that stretch for days, feline eyes and a laugh that sets you free. Sorry, she’s quite the standout.

But more important for “3rd Rock from the Sun,” the out-of-this-world-silly NBC sitcom that airs at 8 p.m. tonight, Johnston also is stand-out funny.

She’s funny taking a punch, funny getting thrown downstairs, even funny enjoying a first sneeze: “Oh,” she gasps, looking downright post-orgasmic, “I want another one!”

On “3rd Rock” (created by the husband-and-wife team of Bonnie and Terry Turner), Johnston joins Joseph-Gordon Levitt, French Stewart and the distinguished actor John Lithgow as a four-member investigative team sent from a far galaxy to learn about human civilization.

There is much to explore here on earth, ranging from the mysteries of Jell-O to the blessings of cable TV to our heroes’ own physicality, which keeps them constantly on the toes they never had before.

But it is Johnston’s character, a courageous military hero back home, who is given a particularly challenging assignment: to learn about female humans by being one.

“We say Sally is a man in a woman’s body, but that’s not technically correct,” says Johnston. “Actually, she is a very masculine, asexual alien inside a woman’s body.”

And, as Johnston is quick to note, not totally unhappy to be there.

Sally Solomon is, if you will, a full-bodied role, by turns goofy, coquettish and combative, tickling America’s funny bone while tweaking its libido.

Johnston reports that women love Sally’s stand-her-ground nature. And while something about Sally makes some men uncomfortable, “judging from the letters I get from prisoners,” Johnston says, rolling her eyes, “it excites them, too.”