Leaderboards
Post a real round once it drains at least five bubbles, under a nickname and, if you like, a state or country. Within 10 and within 20 each keep their own board, with the top 100 counts of the day, the week, the month, and the year. A word filter checks every nickname, and the game never asks for a real name, a town, or an email address. Practice rounds stay on your device.
Top 100 rounds on the Within 10 subtraction facts, most bubbles popped first. Resets at midnight, US Eastern time.
No rounds on this board today yet. Pop some bubbles and post the first score.
How to Play
Choose within 10 for single-digit differences or within 20 for the teen minuends, then press Play or the Enter key. Seven bubbles float up in two rows, and one of them glows — that is the fact to answer. Type its difference; there is no bubble to choose and no Enter key between presses, the game judges the moment you have typed as many digits as the answer needs. Pop it and the water drops a step, the glow jumps to the next bubble in line, and a fresh bubble floats up a moment later to fill the gap.
Here is exactly what to type. Each key below is one press.
One digit. Press 5 and the glowing bubble pops at once.
Two digits, the only two-digit answer in the within-20 set. The game waits for both before it judges.
One digit is enough to judge a one-digit answer. Six is not five, so the water rises a touch, the streak resets, and the same bubble keeps glowing.
- Drain the tank to clear a stage
Answer the glowing bubble fast enough and the water keeps dropping until it hits the floor; the plug opens, the stage clears, and the tank refills a little higher with a faster pump.
- Three in a row drains further
Pop three glowing bubbles without a miss and the water falls an extra step. A wrong answer adds a splash and starts the streak over.
- The golden bubble
After five pops, a golden bubble appears now and then. Answer it while it glows and every bubble in the tank pops at once.
- The water speeds up
Left alone, the tank floods in about half a minute, and the pump runs faster with every minute and every stage cleared, so no round lasts forever.
Why the Bubble Glows
Subtraction answers are short — most differences within 10 or within 20 fit in a single digit — so seven bubbles floating at once would often share the same first key, and the game would have no way to know which one you meant. The glow solves that: only the oldest bubble is live, and typing its difference is the only input the tank accepts.
That is different from the multiplication Bubble Tank, where products rarely repeat a first digit and popping any bubble you recognize is part of the fun. Here the glow keeps the target of practice narrow on purpose: one fact, answered as fast as you can retrieve it, before the next one takes its place.
Keyboard, Keypad, and Sound
Type with the number row or the numeric keypad on a computer; each physical key press lights the matching key on the on-screen keypad so a child watching can follow along. On a phone or tablet, tap the on-screen keypad directly — the same one Math Builders uses during practice, so there is nothing new to learn. The game never opens the device's own keyboard, keeping the tank in view while you type.
Short synthesized cues mark a pop, a golden pop, a drain, a miss, and the danger line; the speaker icon on the card mutes them all and remembers the choice on that device. Nothing plays until Play is pressed.
Using Subtraction Bubble Tank in Class or at Home
On a projector, the glowing bubble is easy to spot from the back of a room, which makes this a workable call-out game: one student holds the keyboard, the rest call out the difference before it is typed. Within 10 with practice on suits a first introduction; within 20 with the water running is where most first and second graders will spend their time once the smaller set is automatic.
The game is not a diagnostic — it rewards a fact already fast, not one still missing. For that groundwork, printable subtraction worksheets and the addition and subtraction fluency page cover how the two skills build together.
Ready for a different kind of pressure, try the Subtraction Speed Grid, where the same facts fill a shuffled grid ahead of a chasing snake, or see how the multiplication version of this game handles a wider table. Every subtraction game lives on the subtraction games hub, and every operation's games are on free math games.
Frequently Asked Questions
Keep answering the glowing bubble fast enough that the water drops all the way to the floor before it reaches the glass lid. Draining the tank clears the stage; it refills a little higher with a faster pump for the next one. There is no final win, only a growing score of bubbles popped and stages cleared.
Only the one that glows. It is always the oldest bubble on screen, and typing digits does nothing to any other bubble until that one is popped.
The water rises a little, your three-in-a-row streak resets, and the same bubble keeps glowing until you answer it correctly. Backspace removes a digit you have already typed, as long as the bubble has not been judged yet.
A rare bubble that can appear once you have popped at least five. Answer it while it glows and every bubble in the tank pops with it, each one adding to your score.
It rides the surface under the tank's glass lid. As the water climbs, the tank's edges glow red as a warning; reach the lid and the duck goes under, upside down, and the round ends.
Yes. Turn on Practice before pressing Play and the water rises at about a third of the normal pace and never reaches the lid. End a practice round anytime with the flag button. Practice rounds are never posted to a leaderboard.
After a real round drains at least five bubbles, post your count under a nickname, with an optional state or country. Within 10 and within 20 each keep their own top 100 for the day, the week, the month, and the year in US Eastern time.