Owls.

Owls share one tree. Every branch holds one owl, and every owl keeps the day or the night. Each row and each column has as many day owls as night owls, no three owls of the same kind sit side by side in a row or a column, and no two rows are alike, nor any two columns.

The day owl is the pale one with a round head; the night owl is the darker one with two ear tufts. The owls that came with the tree sit on a short branch line and cannot be moved.

Every tree has exactly one way to fill it, and it can always be reasoned out, never guessed. When the last branch is right, the game notices by itself.

Tap or click an empty branch for a day owl, again for a night owl, once more to clear it. Hold and drag to put the same owl on several empty branches at once; a drag never turns an owl into the other kind, and a drag that starts by clearing a branch clears the owls it passes. A right click, or Shift and a click, goes one step back. Nothing lights up while you play: finding a mistake is part of it.

Check first tells you only how many owls are wrong and in which rows; press it again and it marks them with a cross and a dashed frame. Hint outlines the row or column to look at and names the step without saying which owl goes where; the second press places the owls and explains why. On a Sunday a hint may try one owl and follow it step by step until something breaks: the second press names every step, numbers the empty branches each step would fill and outlines the line that breaks. If an owl on the tree is already wrong, the hint points there first. Hints and checks are free and counted, and shown when you finish. Clear takes every owl you placed off the tree; the clock keeps running and one Undo brings them back. Share shows up once the tree is solved and copies a short line with the day, your time and the help you used, nothing that gives the tree away.

The counts beside the rows and under the columns say how many day owls and night owls each line holds right now, yours and the given ones together; a line that keeps every rule goes quiet. They never look at the answer. On a very narrow screen they move under the tree, for the row and column you are on.

Keyboard: arrows move from branch to branch, Space or Enter goes around empty, day owl and night owl, D or 1 places a day owl, N or 2 a night owl, Delete, Backspace or 0 clears, Escape lets go, U or Ctrl+Z undo, R, Ctrl+Y or Ctrl+Shift+Z redo, P pauses.

The clock runs from your first owl to the last branch and pauses when you leave the tab or sit still for a minute, which also hides the tree. Your progress, time, history and day streak stay in this browser, and in your account when you are signed in, so you can continue on another device. The day turns over at midnight, Bratislava time, so everyone gets the same tree on the same day.

Monday and Tuesday are easy (six by six), Wednesday and Thursday medium (eight by eight), Friday and Saturday hard (ten by ten), and Sunday is the challenge: twelve by twelve. The week strip under the tree takes you to the other days; the archive goes back a year, and the practice sets have eight trees of each level with no date attached.

Difficulty is measured, not guessed. Every tree is solved by a program that only knows named steps, in three layers: the plain ones (two alike side by side, one empty branch between two alike, a row that already has all its owls of one kind), the ones that read a whole row (every way to finish it agrees on a branch, and no row may become a copy of a finished one), and the hard one (trying one owl and following it until something breaks, which on every tree happens within six steps). Easy needs only the first layer, Medium and Hard never need the third, and on Sunday it comes up now and then.

Start with the pairs. Two owls of one kind side by side always have the other kind at both ends, and one empty branch between two alike always takes the other kind.

Count. A row of six holds exactly three owls of each kind, so once it has its three day owls, every other branch in it is a night owl.

Look for the one that is missing. A row that needs just one more owl of a kind can only take it where the rest does not end up three alike side by side.

From Wednesday on, watch for twins. A row that matches a finished row everywhere except two empty branches must fill them the other way round.