So popular, so quickly has Arena Football become, one already imagines a spinoff: living-room football.

For a modest fee, two-man teams will drop by the house at your convenience. They will remove furniture and batten down pictures, line the carpet and then go flailing at one another. First team to stuff a football into the fireplace 25 times wins.

The possibilities are infinite. Driveway football, since concrete can't be a whole lot harder than what our beloved Commandos and the dastardly Denver Dynamite collided on in Capital Centre Saturday night.

In case you missed it, a crowd of 13,507 watched the Commandos' home opener. The audience was larger, the cheering heartier than at most Bullets' games. Also, the setting was more intimate than any football game you ever have seen.

The only sustained booing was for announcements that fans should return any footballs that strayed from the field. But if customers who caught a batted pass or wild kick decided to be contrary, nobody forced the issue.

If the hitting often seems first rate, the arena game isn't close to real football. Still, it's fun. Once-a-month fun, like a movie with lots of bizarre twists that does not require much thinking. The NFL is religion; Arenaball is pop art.

Probably, many have played a form of Arenaball for years but were unaware. It's fancy sandlot football, a variety of pickup without the stones.

"My profession is {assistant} college football coach," said the Washington quarterback, Richard Ingold, just to make sure everybody knows the level of his game.

Arenaball's season is shorter than the Ivy League's; its pay scale ($500 per game per player, with $100 bonus to each winning starter) might be less than most teams' in the Southwest Conference.

The concept comes off like a Bob Newhart skit. Three quarters through a game, your mind sees Newhart, fist pressed against his ear, ringing up Vince Lombardi and the conversation including such as:

". . . And you know what these teams do when they're backed up inside the 5-yard line, coach?"

"What?"

"They try a field goal."

"You're joking."

"No, because the whole field's only 50 yards long."

"What about running? What about my Packer sweep, which usually gained several yards but always ran lots of time off the clock?"

"Nobody runs much. And except for the final minute of each half, the clock keeps going on incompletions. This encourages victory-conscious coaches to stay entertaining."

"You say a team gets one point for an extra point kicked from placement and two points for a dropkick? And that field goals from placement are three points and four points if dropkicked? How come?"

"Beats me. But try and explain three strikes and four balls to somebody unfamiliar with baseball. Besides, the really complicated stuff involves the nets."

"Nets! Nuts . . . "

Yes, nets. Four of them, two on each end and strung from the goalpost uprights almost to the sidelines. I was worrying how to describe them, until a fellow in front of me leaned toward his buddy and said:

"You wonder where the trampolines in Montgomery County went? There's where they went."

The nets are in play. That gives Arena Football a baseball touch. Players rattling off the boards on tackles affect a hockey mood. And Chief Zee and a couple of Redskins in attendance are reminders of the game's roots.

The father of Arena Football, Jim Foster, evidently realizes almost nobody understands defense. College or pro. So his eight-man teams don't play any. Or anything very complex.

Washington stuck to basics. You know, the traditional 3-2-2-1. Like the NBA, zones are not allowed but probably used anyway. Lombardi and Sam Huff would be furious.

"I was surprised the score was this low," said Washington Coach Bob Harrison. Only 56 points total, with the Commandos getting 14 the first three minutes and 36 in all.

Because the field is so constricted, players have less chance to generate the sort of frightening momentum that causes so much injury in real football.

The possibility of fairly injury-free football was comforting only until the Denver quarterback was carried off the field shortly before halftime. Later, it was revealed that an appealing Commando, Richard DuPree, might miss the rest of the season with a knee injury.

Football, it develops, is unsafe at any speed.

Arenaball will survive as long as it does not take itself too seriously. Or expect us to do the same. The pretentious who consider this an affront to football ought to consider a few facts:

Ingold is in about the same position as Sammy Baugh and most Redskins were when they arrived here in 1937. It was decades before the NFL evolved into more than a part-time job.

"We're in the sports entertainment business," Foster kept saying. Over and over. And still over once more early Sunday, until it appeared that a couple of antsy cleaning women were about to bop him with their mops.

The league is learning about itself and its potential; the players are examining how those nets can be used for points and profit. Consider this intriguing rule:

"A forward pass that rebounds off the nets within the field of play is a live ball and is playable until it touches the playing surface."

That jump-starts the imagination.

Let's say Lenny Taylor or another Commando gets a half-step on a defender. The easiest way to a touchdown just might be for Ingold to wing the ball far over Taylor's head and off one of the nets downfield, hoping that it will angle back and meet the receiver in full and uninterrupted stride.

"Or," said Harrison, getting into full Arenaball stride, "a receiver could run a defender into the end and stop. Quickly. The defender would keep going a stride or so. Meanwhile, the receiver would look up and -- whee! -- catch the ball off the net {for a touchdown}."

That might be future shock. Fans Saturday night had fun watching the Commandos mascot drop from the ceiling before the game; they were amused by the five guys spelling out Commandos with their bodies; they were thrilled by a defensive stand early in the fourth quarter.

A decently solid cult group might have left smiling and saying: "How 'bout them 'Dos?"