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Hourglass Puzzle is a free online version of the classic sand-timer puzzle. You have two or three hourglasses, say a 7-minute and an 11-minute glass, and no other clock. You may only act at the start and at the moment a glass runs out: turn any glasses over, put the egg in, then let the sand run to the next glass running out. Measure exactly 15 minutes, or 9 with a 4 and a 7, by turning a glass over part-way through so its fallen sand runs back. Twelve levels go from two minutes of tea to a three-glass finale that no pair could manage alone. A computer search sets par in turns for every level, Hint names what to do now, and a minute of sand passes in a fifth of a second, so nobody waits. No download, no sign-up, works on phones.
How to play Hourglass Puzzle
- Each level gives you two or three sand glasses, each marked with the minutes of sand it holds, and a goal such as "boil exactly 15 minutes, counted from the start". On some levels you choose when the egg goes in.
- Tap a glass to turn it over. A glass runs while it has sand in its top bulb; the label above it shows how many minutes are left on top.
- Press Let It Run and the sand runs until the next glass runs out — a minute of sand passes in a fifth of a second. That moment is the only time you can act again, because there is no other clock.
- Turning over a glass that is part-way through puts the sand that has already fallen back on top. That is the trick behind most levels: a 7-minute glass with 4 minutes fallen becomes a 4-minute timer.
- On levels that let you choose, press Egg In at any moment you can act; the boil is timed from then. The level is won the moment a glass runs out with the egg boiled exactly the right number of minutes.
- Every turn of a glass counts. Finish in par — the fewest turns possible — for three stars, within the two-star line for two, and any finish earns one. The game will not let the sand run past the boil; Undo and Start Again take you back, and Hint says what to do now.
- On a keyboard, number keys turn glasses, R lets the sand run, E puts the egg in, U undoes and H gives a hint.
Tips
- Write down the minute each glass will run out. Every moment you can act is one of those minutes, so the answer is a sum or a difference of them.
- Starting both glasses at once and waiting for the short one is almost always the first move: it makes the difference between the two glasses appear as sand in the long one.
- The fallen sand in a part-run glass is a timer you have already paid for. When one glass runs out, look at what has fallen in the other and ask whether running it back gives the minute you need.
- On levels where you choose when the egg goes in, the boil does not have to start at minute 0 — put it in when a glass runs out and let the difference between two glasses do the timing.
- If the next glass would run out after the egg is done, turn something over first so a glass runs out sooner. If nothing can, the boil has gone wrong earlier: ask for a Hint and it will say how far to undo.
FAQ
Is Hourglass Puzzle free to play?
Yes. It runs in the browser with no download, no sign-up and nothing to buy. All twelve levels are included and unlock one after another as you finish them.
How do you measure 15 minutes with a 7-minute and an 11-minute hourglass?
Start both glasses together. When the 7-minute glass runs out, turn it over. When the 11-minute glass runs out at minute 11, the 7-minute glass has run for 4 minutes, so turn it over again: those 4 minutes of sand run back and it runs out at exactly minute 15. That is four turns, and no way uses fewer. It is level 4.
How do you measure 9 minutes with a 4-minute and a 7-minute hourglass?
Start both. At minute 4 turn the 4-minute glass over. At minute 7 the 7-minute glass runs out; turn it over. At minute 8 the 4-minute glass runs out, and the 7-minute glass has 1 minute of fallen sand, so turn it over and it runs out at exactly minute 9. That is five turns, the fewest possible. It is level 5.
When am I allowed to turn a glass over?
Only at the start and at the moment a glass runs out. The glasses are the only clock you have, so those are the only moments you can know the time. You may turn any number of glasses at once, and the egg can go in at any of those moments too.
Do I have to wait real minutes?
No. The sand runs fast: each minute of sand takes a fifth of a second on screen, and no run takes longer than about two seconds. The clock on the shelf shows the real minute you have reached.
How are par and the stars worked out?
A computer search tries every position a level can reach and finds the fewest turns that time the boil exactly, so three stars are always possible. The two-star line is the median of 41 runs by a careful trial-and-error player that spots a finish two runs ahead and never lets the sand overboil. Any finish earns one star.
Can I get stuck?
Yes, but never for long. If the egg goes in at a moment from which no turns can hit the minute, the boil can no longer come out right. The game will not let the sand run past the boil, and Hint then tells you how many steps to undo. Undo has no limit, and Start Again resets the level.
Does Hourglass Puzzle work on a phone?
Yes. Tap a glass to turn it and tap Let It Run. On a phone held upright the glasses sit above the goal and the buttons; on a wider screen the goal and the buttons sit beside them.