The big idea
Every law in physics was discovered by someone measuring something carefully. Before you can work out speeds, densities or energies, you need reliable numbers for three basic things: length, volume and time. The tools are simple. The skill is in using them well.
Explained simply
Length means how long something is. In physics we measure it in metres (m), but small things are often measured in centimetres (cm) or millimetres (mm). Use a ruler for short lengths and a tape measure for long ones like the length of a corridor. Always line up the zero mark with the start of the object, and look straight down at the scale. Looking from an angle gives a wrong reading called a parallax error, which means the reading shifts because your eye is not directly in line with the mark.
Volume means how much space something takes up. For a liquid, pour it into a measuring cylinder, which is a tall, narrow container with a scale up the side. Water curves slightly at the surface, forming a meniscus. Read the scale at the bottom of this curve, with your eye level with it. Volumes are measured in cm³ or m³. One millilitre (ml) is the same as 1 cm³.