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The 9 Minute Egg

Boy, this was a tough one! Based on the responses we received, this puzzle rivals The Magnet for "simplest puzzle with toughest solution". The trouble was not timing the egg, lots of people could handle that. The hard part was finding the shortest amount of time, which was as hard as this egg after boiling for nine minutes. We received (incorrect) answers as long as nearly a half hour.

How long does the professor have to wait? As little time as it would take if the clocks were working- nine minutes.

First, flip both hourglasses over and drop the egg into the water. (Use your third hand, balance the egg on one hourglass, whatever.) When the four minute timer runs out, flip it again. When the seven minute timer runs out, flip it over. The egg has been cooking seven minutes.

Now when the four minute timer runs out again (after eight minutes) flip the seven minute timer back over. Since the seven minute timer has been running only a minute between flips, there's a minute worth of sand left. And when that minute runs out, the egg will have been cooking for exactly nine minutes.

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