The dates 4/4, 6/6, 10/10 and 12/12 all fall on the same day of the week every year. They're called doomsdays.

More in "Did You Know?" Series

• Your body is the weakest during 3 am - 4 am. This is the time most people die in their sleep.
• One reason men cry less than women is because they have larger tear ducts, preventing their eyes from leaking easily.
• Research has shown that people are happiest at 7:26pm on Saturday evening.
• Water is the only substance that is found naturally on Earth in three forms - liquid, solid and gas.
• There is a type of tree called rainbow eucalyptus tree, whose bark naturally contains various colors.
• To protect paintings from rodents, around 70 cats work at the Hermitage.
• Reindeers are so well-adapted to the Arctic winter, they are more likely to overheat than freeze in the cold.
• According to one survey, men spend nearly a year of their life staring at women.
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