Explaining Why
It rained in the night, and in the morning there is a big puddle in the playground. By the afternoon, after a sunny morning, it has gone. Where did the water go? "It just went" is true, but it isn't an answer. What does it take to explain something?
An explanation says how
Here is an explanation. The water didn't vanish. It slowly turned into a gas, called water vapour, which is invisible, and mixed into the air. Water turning into vapour like this is called evaporation. It happens faster when the water is warm and when the wind carries the damp air away.
An explanation says how or why something happens: what goes on, step by step, that makes it happen. "It just went" says nothing about how. "It turned into vapour and mixed into the air" does.
One idea, many things
A good explanation does more than explain the one thing you asked about. Think of all the things evaporation also explains: wet washing dries on a line, and faster on a windy day. Wet hair dries. When sea water is left in shallow pools in the sun, the water goes and salt is left behind - which is how sea salt has been made for thousands of years.
One idea, many things explained. That is what makes scientists trust an explanation more: it isn't made up for one case, it fits many cases that seemed to have nothing to do with each other.
It can be tested
A good explanation also makes predictions, so it can be checked, like the hypotheses in the If It Is True, Then... lesson. If evaporation explains the puddle, then a dish of water covered with a lid should keep its water for longer than an open one. A dish in the sun should dry faster than one in the shade. You can try both.
Try it at home
- Put the same amount of water in two cups or saucers. Cover one with a plate.
- Put a mark at the water line of each. Predict what will happen.
- Check every day for a week and write down what you see. Does the explanation pass the test?
Words to know
- explanation - an account of how or why something happens
- evaporation - water turning into an invisible gas, water vapour, and mixing into the air
- water vapour - water as an invisible gas in the air
Big idea
An explanation says how something happens. A good one explains many different things at once - puddles, washing, sea salt - and makes predictions you can test.