Marble Sort Level 21 greets you with a wall of unknowns. A 3x3 slab of Mystery Boxes sits dead centre, a Tunnel stamped 3 guards each flank, and the colours you can actually touch are Blue, Black, Green, and one Red that's nothing but trouble. The whole level is a countdown: clear three Boxes and both Tunnels pop, spilling the colours you'll need later.
The front row reads Black, Green, Blue, Red, Black, and Blue, left to right. Behind the left Tunnel hides a stack of three Light Blues and a Pink. Behind the right one sit a Green, two Browns, and another Green, with a stray Brown-Green-Blue column hanging off the far edge.
Marble Sort Level 21 Walkthrough
Your bottom columns set the rhythm. Column one runs Blue, Yellow, Pink, Light Blue, Green. Column two is Blue, Yellow, Brown, Black, Yellow. Column three is Black, Green, Brown, Pink, Yellow. Column four is Green, Black, Green, Green, Pink. The active front is two Blues on the left, a Black, and a Green on the right. That's the disconnect: a Red block sits right in your face, but there isn't a Red Box anywhere near the top. Touch it and it camps on the belt for the whole round.
How to Beat Marble Sort Level 21 — Step-by-Step
Unlock the Tunnels
- Tap both Blue blocks in the front row; they drop straight into the two Blue Boxes on the left, and both Tunnels tick down to 1.
- Tap the Black block on the far left, then the Green block sitting above the Green Box.
- Both marbles land in their active Boxes, and the left and right Tunnels spring open.
- Keep ignoring the Red. It stays bait.
Open the Mystery grid
- The front line is gone, so the 3x3 Mystery grid is now in reach. Drop the grid's bottom row.
- Your active targets flip to Yellow, Yellow, Green, and Black.
- Read each reveal. If a Mystery Box spits out Yellow, Green, or Black, match it straight away to keep things flowing.
- The right Tunnel is now feeding Green and Brown. Column four has a Green Box, so route those Greens into it.
- Keep a close eye on the belt; only pull down a revealed colour that fits the current front row.
Push the final layers
- The columns start wanting Brown, Pink, and Light Blue.
- The left Tunnel holds your Light Blue and Pink supply, so pull from the left flank to clear columns one and three.
- The right flank and that staggered far-right column hold the Brown, so use those for the middle layers of columns two and three.
- By the time a Red Box finally surfaces, drop that patient front Red block and match the rest patiently to close the board.
Tips & tricks
- Red is the decoy of the level; it has no Box until the final stretch, so leave it where it is.
- Both Tunnels need three clears, and the two Blues plus the Black and Green are exactly enough to open them.
- Match every Mystery reveal to the current front row or let it sit — a dead drop here is what floods the belt.
Common mistakes
- Touching the front Red. It parks on the belt for the entire midgame and starves you of space.
- Pulling a Mystery colour before its Box is active, which strands it on a belt that's already tight.
Marble Sort Level 21 FAQ
Why does my Conveyor Belt flood on Marble Sort Level 21?
You touched the front Red block, which has no Box until the late game, so it sits on the belt and eats space. Leave the Red alone and clear the two Blues, then the Black and Green.
Which color do I clear first on Marble Sort Level 21?
The two Blue blocks in the front row. They clear both Blue Boxes and drop the Tunnels to one, setting up the Black and Green clears that open them.
How many moves does it take to clear Marble Sort Level 21?
Around twelve taps across three phases — the Tunnel unlock, the Mystery grid reveals, then the Brown, Pink, and Light Blue cleanup with the Red held for last.
Do I need boosters to beat Marble Sort Level 21?
No. Open the Tunnels with the exact front-row clears and the board supplies everything else. Save boosters for boards with hidden Boxes or locks.
What happens if I tap the Red block early?
It drops onto the Conveyor Belt with no Red Box to receive it, so it camps there for half the level and chokes the space you need for the Mystery reveals.
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