Central principles
 

Conservation in measurement

Next we should note the principle of conservation, another fundamental idea in learning about measurement of length, mass and liquid volume. Pupils meet this principle first in the context of conservation of number. They have to learn, for example, that if you rearrange a set of counters in different ways you do not alter the number of counters.

Similarly, if two objects are the same length, they remain the same length when one is moved to a new position: this is the principle of conservation of length.

Conservation of mass is experienced when pupils balance two lumps of dough, and then rearrange each lump in some way, such as breaking one up into small pieces and moulding the other into some shape or other, and then checking that they still balance.

Conservation of liquid volume is the one that catches many pupils out. When they empty the water from one container into another, differently shaped container, as shown in my diagram, by focusing their attention on the heights of the water in the containers, pupils often lose their grip on the principle that the volume of water has actually been unchanged by the transformation.