Metres or centimetres?

In your book, Maths Mela (Chapter 5, Far and Near), we learn more units for measuring length and distance. First, a question: when do we use metres (m), and when centimetres (cm)?

A metre is long: about as high as a door handle is from the floor. We use metres for big things, like the height of a building, the length of a room or a playground. A centimetre is tiny: about the width of your little finger. We use centimetres for small things, like a pencil, a book or a phone. 100 cm make 1 m.

The book gives numbers without units, and we must choose. The height of India Gate is 42 — that must be 42 m, because 42 cm is shorter than your arm. A handkerchief is 40 cm long, not 40 m, which would be longer than a row of houses. A mobile phone is 13 cm. An elephant's trunk is 2 m long, and the gap between two buttons on a shirt is 5 cm.

So always think: does this number make sense with this unit? The same number can be very long in metres and very short in centimetres.