My little sister was bored the other day. It was evening, raining outside, nothing interesting on TV. So she started to play with me. She asked tricky questions.

'What is that? I always fall but I'm never hurt.' I laughed. 'It must be GUESS.

'Okay', she said, 'I have a more difficult one: I can run without feet.'

I was thinking hard. She helped me: 'it is about water, too.'

'Oh, it is a GUESS then', I replied. The next turn was mine.

'What is that? I have a bed, but don't sleep.' She said, 'either GUESS or a GUESS , as both have beds.' How smart my sister is!

Now I wanted to ask her a difficult question, so I asked: 'my chemical formula is H₂O. What am I?'

She got upset: 'It is not fair. You go to secondary school, but I don’t. What is a ‘chemical formula’? What is this symbol?'

I explained that water in fact is a combination of two elements, oxygen and hydrogen.

'You lie! ‘ She shouted at me. 'Even I know that oxygen and hydrogen are gases. They are not liquid. How can you say that oxygen runs out of our tap?'

'Do you know that we breathe oxygen from the air? Oxygen is needed when things burn. You just burn some hydrogen gas, and it will turn into water.'

'Really?' She brought me a glass of water. 'Please take hydrogen out of it and burn it for me. I want to see it.'

'Sorry, I can't do that', I said. 'It is very difficult to take hydrogen out of water. Hydrogen and oxygen hold each other very tightly.'

She looked at the glass of water closely. 'I don’t see anything in it. I don’t see oxygen or hydrogen in it. It is just liquid. '

'You know how small they are? There are billions of billions of pairs of oxygen and hydrogen in it.'

'Pairs? Are they in pairs?'

'No, I was wrong. Every oxygen holds two hydrogens. Hydrogens are much smaller than oxygen. Something like this.'

I drew the formula and a water molecule.

'Yeee', she laughed, 'they are like a mother with two kids'.

'Yes, and the mother doesn’t let the children run away. This is water. But in reality, oxygen and hydrogen have no legs, no eyes.'

• Do you know where oxygen comes from? Answer
• Do you know what the use of hydrogen is? Answer
• What does the number two mean in the chemical formula? Answer
• Do you think it is important to know what elements are there in water? Why? Why not?