You are a cook in a remote area with no clocks or other way of keeping time other than a four-minute and a seven-minute hourglass. On the stove is a pot of boiling water. Jill asks you to cook a nine-minute egg in exactly 9 minutes, and you know she is a perfectionist and can tell if you under cook or overcook the egg by even a few seconds. How can you cook the egg for exactly 9 minutes?
1. Flip both hourglasses over and drop the egg into the water. 2. When the 4-minute timer runs out, flip it over (4 minutes elapsed, 3 remaining on the 7-minute timer). 3. When the 7-minute timer runs out, flip it over. (7 minutes elapsed, 1 remaining in the 4-minute timer) 4. When the 4-minute timer runs out, flip the 7-minute timer over. (8 minutes elapsed. 6 minutes remained in the 7-minute timer, but flipping it over leaves one minute’s worth of sand on top. When it runs out exactly nine minutes will have elapsed.)
Her birthday is December 31st. Today is January 1st so she was 7 two days ago, and turned 7 last year. Now she’s 8 and will turn 9 this year. And next year she’ll turn 10.
For example, if today is 01 Jan 2015:
30 Dec 2014 = age 7 (two days ago) 01 Jan 2015 = age 8 (today, she turned 8 on 31 Dec 2014, which was last year) 31 Dec 2015 = age 9 (her birthday this year) 31 Dec 2016 = age 10 (her birthday next year)
What must be in the oven yet can not be baked? Grows in the heat yet shuns the light of day? What sinks in water but rises with air? Looks like sand, but is fine as hair?
