The floor is lava. You're the lava.

The kids can see you coming

Milo is the dozy one. On your first go he doesn't dodge, and holding under him gets him OUT in about a second. That's the game teaching you the hold.

Then it tells you "THE OTHERS DODGE." It means it. The kids watch the floor. When the lava humps up near them, or a crest is heading their way, they hop to the safest spot they can reach.

I checked how far a still thumb gets you. I held one under every kid in every room, 40 times each, on the game's own code, for five seconds. Kids caught: zero. The one exception is June on the garden trampoline. She can't stop bouncing long enough to get away, so she goes 40 times out of 40.

Chasing a kid with the swell (the hump of lava your thumb pulls up) works when they've got nowhere higher to hop. Milo on the armchair: caught every time. June on the sofa: never, because the sofa back is right behind her.

Letting go right under a kid does better than holding still (numbers further down), but it won't get you through the garden. So you throw. Hold, drag, let go.

Drag it at the speed of a wave

A wave crosses the room in about two seconds. Drag your swell along at that same pace and it rides forward and keeps growing. SURFING! pops up over it while you've got the pace right. Then let go before you reach the kid, so the crest arrives under their feet.

The game's coach tells you DRAG FAST TO SURF A BIGGER WAVE. Fast means wave speed, not flat out. Whip your thumb across at twice that and the far end of the room barely notices.

Speed gets the lava up there. Where you let go decides whether the kid is still standing on it when it arrives. I tested that on June, put up on the living-room bookshelf, and the numbers are further down.

A few more rules you'll see on screen:

• A crest that slams a wall jumps to double height right at the wall, then heads back the way it came. The game shakes, and a big one gets a SPLOOSH!
• Two waves that cross add together.
• A kid caught mid-hop is MID-AIR!, double points.
• OUTs less than 0.7 seconds apart chain a combo, and the combo multiplies the points.
• A very tall crest spills over the top, and the level settles back to normal, so you can't flood the house for good.

One Saturday afternoon, five rooms

You play the rooms in order. Most rooms add a kid, and every room's kids react faster than the last: they wait 0.45 seconds before dodging in the living room and 0.15 in the garden.

1. THE LIVING ROOM. Milo and June. June's on the sofa with the sofa back right behind her, and the bookshelf is 222 up.
2. THE PLAYROOM. Three kids. June starts on the top bunk, 210 up, nearly as high as that bookshelf.
3. THE KITCHEN. Three kids, quicker. Oskar starts on the counter, one hop from the cabinet above it, and the fridge is the tallest thing in the house so far.
4. MUM AND DAD'S BED. Four kids. The mattress is wide and low, and it's the one room where letting go right under them beats throwing.
5. THE BACK GARDEN. Five. June's on the trampoline and can't stop bouncing. Nan's up the tree, 236 up.

The garden is the wall. Started fresh in the garden, my best bots clear it 10 to 18 percent of the time. I haven't worked out a reliable way to get Nan.

One dinner clock for the whole afternoon

You start with 40 seconds until dinner, and that one clock runs across all five rooms. Every OUT buys 3 seconds. Every room you clear buys 8 more, and the banner says MUM SAYS +8 SECONDS. When it hits zero: DINNER'S READY! EVERYONE OFF THE FLOOR.

The card at the end shows the rooms you reached and hides the rooms you've never reached as ???. If you lose with one kid left, it says so: ONE KID LEFT. SO CLOSE. One tap and you're back in the living room with the clock running. A good afternoon takes about two minutes.

What I measured, and what nobody has done yet

The numbers on this page come from bots I wrote to play the game. They run the same code you just played. They're not people.

• Holding under a kid and letting go works better than you'd guess. A bot that does only that clears each of the first four rooms 58 to 83 percent of the time from a fresh start. The garden is where it stops: 3 percent.
• Over a whole afternoon the throw still wins. Across 40 runs, a bot that drags and throws (with its aim made deliberately late and sloppy) averages 5,331 points and clears at least three rooms every time. The hold-and-let-go bot averages 1,314 and clears nothing at all in 15 of the 40.
• One result surprised me. On Mum and Dad's bed, hold-and-let-go clears the room more often (75 percent) than the throwing bots (55 to 63 percent). The bed is wide and low. I left it in.

The bookshelf test from earlier. June stands on the living-room bookshelf, 222 up in the game's units (the sofa back is 138). Same swell every time: held 0.7 seconds at the armchair end, then dragged toward her at different speeds. The first column is the tallest the lava gets at the shelf with nobody on it. The other two count how often June goes OUT in 40 tries, letting go two thirds of the way across or just short of the shelf.

Only a drag near wave speed gets her at all, and which one works depends on where you let go. The 1.4 times drag is past where SURFING! shows, and let go late it gets her every time: she never hops. Most of the misses at wave speed are June seeing it coming and hopping down. The kids can see you coming.

Here's what hasn't happened. Nobody has playtested it on a phone with a real thumb yet. The touch checks so far are an automated run that drives real touch input in a phone-sized window. Nobody has sat and listened to the sound either: the kid voices, the rumble and the drone are all made in your browser, and there's no music yet. There aren't any players yet. You'd be one of the first.

Questions

Do I need to install anything?

No. It runs in the browser, with no account and no ads. It remembers your best score, which rooms you've seen and whether you've had the tutorial, in this browser only. Nothing gets sent anywhere.

Does it work on a phone?

It's built for one thumb, but as above, no person has played it on one yet. Upright works. Sideways gives you a bigger room, and the game tells you so.

What does OUT mean? Kids falling in lava?

They're playing floor is lava, and you're the floor. OUT means what it means on the sofa cushions: you touched the floor, you're out. They yell WHOA! when you nearly get them and NYAH NYAH! when they dodge. An OUT is a puff of steam and their name in big letters. The picture under "The kids can see you coming" is exactly what it looks like.

How long is a game?

About two minutes for an afternoon. Less if the kitchen gets you.

Can I use a mouse?

Yes, it's the same as a thumb: press, drag, let go. R starts over and M mutes.

How does the lava actually work?

It's a wave equation in one dimension, with three guards so it can't run away. That's why a dragged crest travels as one piece, jumps to double height where it hits a wall, and adds to any wave it crosses. Each trip across the room costs it about half its height. Pulling a swell up drains the floor around it. It's a toy model, not a lava model.