Building Blocks
The STACK method in four minutes — one real question, the blocks turned and slotted into the shape.
A 3D shape sits at the top, solid all the way through with nothing hollow inside. Each option is a set of loose blocks, drawn apart. You pick the set that fits together to build it exactly — nothing left over, no gaps.
The blocks are drawn as solid pieces, with no lines marking the cubes inside them. You judge their size by eye, against the shape.
The video above and the worked example below take apart the same question. One thing is different: the video shows the five options in another order, so the answer is lettered D in the video and A here. Same picture, different place in the line-up.
The method
Five steps: size up the shape, too big to fit, amount, corner first, keep the set that fills it — and their first letters spell STACK, which is how the blocks go together.
- Step 1 of 5
Size up the shape
How long, how wide and how tall is it? Find its longest straight run. - Step 2 of 5
Too big to fit?
A block that is longer, wider or taller than the shape cannot fit, however you turn it. Any set holding one is out. This catches more wrong sets than anything else, and it can never throw out the right one. - Step 3 of 5
Amount: too much or too little?
Roughly, do the blocks hold as many cubes as the shape? A set with a whole block too many is out. - Step 4 of 5
Corner first
Picture the biggest block in a corner of the shape, then fill the rest. A corner pins a block down; big blocks have the fewest places to go. If a gap is left that nothing can fill, the set is out. - Step 5 of 5
Keep the set that fills it
Every gap filled, nothing left over.
Give each question 30 to 40 seconds.
Rules to watch
- Nothing is hollow. Every cube inside the shape has to come from somewhere.
- Blocks can be turned any way at all — on their side, or even upside down. Judge each block by its shape, not by how it is drawn.
- Do not fear a mirrored block. None of the 72 wrong sets in our exam-level bank is the right set with one block mirrored, so rejecting a set for that would lose you marks.
- Counting is rough, not exact. With no lines inside the blocks, use the amount check to cross out a set that is plainly too big, then let the building decide.
- Start with the hardest block. A plain straight brick fits almost anywhere and tells you nothing. A big or bent block tells you nearly everything.
Watch for these traps
We looked at every wrong set in our eighteen exam-level questions — and the app's own packer checked every one.
- A block too big to fit. 31 of the 72: 17 hold a block too long, 14 a block too wide or too tall some other way. Step 2 catches all of them in seconds.
- It nearly fits. 26 of the 72 hold the right amount, with every block small enough to fit somewhere — and still the shape cannot be built. Only steps 4 and 5 find these: you end up with a gap you cannot fill, or a block with no room.
- One block too many. 9 of the 72 are the right set with one more block piled on. A further 6 swap in a block of the wrong size. Step 3 catches all fifteen.
Stuck at 35 seconds? Cross out the sets with a block too big and the wrong amount, then pick from what is left.
Try these yourself
Now have a go yourself — tap the set that builds the shape.
Related
- The complete 11+ question-type guide
- Complete the Shape — find the piece that fills the gap
- 3D Rotation — the same shape turned, and the mirror trap
All example questions come from the Brainblox question bank — practise this question type inside 25+ learning games, timed mock exams and focused practice. Brainblox is an independent app: not affiliated with, endorsed by or licensed by any exam board, school or assessment publisher, and it contains no past papers.