The right unit and tool

The unit should suit the length. Measuring the distance between two cities in metres gives a huge number; measuring the thickness of a page in metres gives a tiny one. So pick a unit that fits.

Your book's examples:

• Distance between Delhi and Lucknow: long, so kilometres.
• Length of a school ground: metres.
• Length of an eraser: centimetres.
• Thickness of a coin: very small, so millimetres.

Pick the right tool too:

• Your pencil: a 15-cm scale is enough.
• The height of a room: a metre scale or a measuring tape.
• The girth of a tree (the length around its trunk) or the size of your chest: a stiff metre scale cannot bend round them. Use a flexible measuring tape, like a tailor's tape, which wraps around the object.