How many liters are in a gallon? If the temperature is 33 degrees Celsius, what is it in degrees Fahrenheit?

If you know the answers, good for you!  But if you can’t remember, don’t feel too bad; most Americans can’t remember either.  That’s because the relationship between measurement systems seems completely random.  There’s no obvious reason why there are 3.8 liters in one gallon, rather than, say, 3.9 or 4.2 liters. Similarly, there’s no obvious reason why water, which boils at 100 degrees Celsius, requires 212 degrees on the Fahrenheit scale.

Random facts are hard to remember.  You can try to memorize them the way school children do.  Or, you can just estimate. You can notice that four liters is a little more than a gallon, and most of the time, that’s good enough.  For example, in the grocery store, it means that three liters of soda for $2 is a better bargain than half a gallon at the same price. Similarly, if you’re on holiday in France, and the radio reports that the temperature on the Cote d’Azur is 35 degrees Celsius, don’t worry about how to convert precisely to Fahrenheit.  If you can just remember that body temperature is about 37 degrees Celsius, then you know that an air temperature of 35 degrees Celsius means it’s going to be a hot day at the beach.

It isn’t just about weights and measures, either – the same principle works for remembering dates.  If have trouble remembering whether your sister-in-law’s birthday is October 12th or October 15th, don’t sweat the details.  Just estimate.  Remember that the big day falls in the same month as Halloween, send your card in early October, and bask in the approval when she calls to tell you your card was the first to arrive.