Daily Numberlink
The rule
Join each pair of matching numbers with a line, and use every square on the board.
Lines move one square at a time, up down left or right. They never cross and never share a square. When you are done there should be no empty square left.
Tap a number to take hold of its line, then tap along to extend it. Tap a square the line already covers to wind it back to there, and tap the number again to drop the line.
Where to start
Corners decide themselves. A corner square has only two ways out, so whichever line reaches it has to turn there, and if a number sits in a corner its line can only leave one way.
Then think about the squares nothing wants. Filling the whole board is the real constraint: a route that looks neat but strands one square in the middle of somebody else's territory is wrong, and spotting that early saves unpicking a lot of line.
In your hands
Threading something through a space with no room to spare is what a packing puzzle asks of you, in three dimensions.
One Answer, and the Board Always Fills
Each board was made by laying the lines down first so they covered every square, then reading the pairs of numbers off their ends. That guarantees an answer exists. The question worth asking is the other one: whether those numbers could be joined some other way. It is settled by counting the routings rather than by seeing whether a solver manages, and only boards with exactly one are kept. Around nineteen in twenty pass, which is high, because filling every square is a far heavier constraint than it looks.
More to Play
There are twenty four daily sudoku variants, a hashiwokakero, a kakuro, a slitherlink and a pentomino board. For puzzles you can hold, see mechanical puzzles.
Fitting shapes into a space with nothing to spare is the same instinct, in wood you can pick up. See the Fish Tank, or the rest of the collection.
Frequently Asked Questions
How do you play Numberlink?
Join each pair of matching numbers with a line, and use every square on the board.
Is Numberlink free to play?
Yes. There is a new numberlink here every day, free, with no account and nothing to install. It plays in the browser on a phone or a computer, and it remembers where you got to.
Can every puzzle be solved?
Yes. The lines are laid down first so they cover the whole board, and only the boards whose numbers admit exactly one routing are kept.