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Another piece is blocking that direction. When you touch a piece, it wobbles slightly toward the directions it can go — trace it that way. If a piece does not wobble at all, move other pieces first. If you are stuck, the light-bulb button (hint) at the bottom lights up the next piece to move.

Hints are available only while you are taking the puzzle apart, not while you are putting it back together.

Trace empty space with your finger and the whole burr turns in any direction. The eye button (see-through) at the bottom makes every piece but the chosen one translucent.

Double-tap empty space to return to the starting view. Pinch with two fingers to zoom in and out.

It stays out — moving the other pieces does not pull it back in. To put the burr back together, return a piece to its original place and it clicks in. Pieces that are back in place block the way for the others, in the order you put them back.

“Undo” at the bottom takes back one move at a time. “Start over” asks first, then returns the puzzle to its assembled state (your progress on that puzzle is lost).

Each puzzle remembers its board, so you can switch to another puzzle and come back to continue where you left off.

They are 3–4 hexadecimal digits. The first one or two digits are the number of pieces (6 = 6, 9 = 9, C = 12, F = 15, 12 = 18, 15 = 21), the next digit is the number of moves in the hardest single stage (F for 16 or more), and the last digit numbers puzzles with the same pieces and moves, easiest first.

It marks a solved puzzle. Gold means you have solved it at least once without hints; silver means you solved it with hints (once gold, it stays gold).

See the FAQ on the top page.