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Join each pair of matching numbers with a line, never crossing, and cover every square on the grid.
Every number on the board appears exactly twice. Drag from one of them to its twin to draw a line between them, moving only up, down, left or right — never diagonally.
Two rules make it a puzzle instead of a doodle. Lines may never cross or share a square, and when you are done every single square must be covered by some line. A board where all the pairs are joined but one square is still empty is not solved, and that second rule is what forces the long detours.
Dragging across a line you have already drawn cuts it back to that point, so you can repair a route without wiping it first. Starting a fresh drag from a number clears that colour and leaves the others alone. The counter under the board tells you how many pairs are joined and how many squares are covered, which is usually enough to see what went wrong.
Every board is generated and then checked by a solver before you see it: among all the ways of joining the pairs, exactly one also fills the grid completely. So there is always precisely one right answer, and no guessing is needed to find it. Your time is your score, so lower is better.
The corners solve themselves and almost nobody looks at them first. A corner square has only two neighbours, so whichever line covers it must enter from one and leave by the other — and if the corner is itself a number, its line's first step is forced. Do every corner before anything else and you will usually have three or four squares decided for free.
The same argument runs along the edges. An edge square that is not a number has three neighbours, and if one of those is already taken by another colour, its route is fixed. This is why experienced players hug the border first and work inward: the walls do the deductions for you, while the middle of the board has four ways out of every square and tells you almost nothing.
Never draw the straight line between a pair just because it fits. Coverage is the real constraint, and the shortest route is almost always wrong precisely because it leaves squares stranded behind it. Before committing a line, glance at what is on the far side of it: if a pocket of empty squares would be cut off with no number able to reach it, that route is dead however neat it looks.
Count parity when you are stuck on the last two colours. Every square must be used exactly once, so the lengths of all the lines add up to the number of squares — thirty-six on the standard board. If you already know the length of three lines, the remaining two are pinned to a total, and that total often rules out every route but one.
Finally, use the cut-back behaviour instead of restarting. Dragging over your own line trims it to where your finger is, so you can back a line out three squares and take the other turn without losing the rest of it. Players who clear a whole colour every time they make a mistake lose more time to redrawing than to thinking.