Students can have a difficult time understanding the distinction between the concepts of mass and weight. In fact, even scientists routinely use these two terms as though they were interchangeable. There is an important difference between the two. You can demonstrate this difference to young learners with a few examples and a hands-on demonstration.
The mass of an object is a simple concept, but it can sometimes be difficult to properly explain. Mass represents the quantity of an object. Mass doesn't always vary proportionately with size, but it normally does. A teacher can explain how a large vehicle like a truck has more mass than a small vehicle like a motorcycle. Mass is often measured in pounds and ounces in imperial units, or in grams and kilograms in the metric system.
One difficulty in teaching the difference between weight and mass is that the two terms are often used to mean the same thing, using the same units. It is very common for someone to say they weigh a certain number of pounds, for example. Strictly speaking, weight is the force with which the Earth's gravity pulls something towards it. Weight should actually be given in units of pound-force or, in the metric system, Newtons.
Mass Without Weight
One way to illustrate the difference between mass and weight is to show students photos or videos of astronauts in outer space. The NASA Shuttle Mission picture gallery has a number of photographs of weightless astronauts. Teachers can explain to students that the astronauts are weightless in space, since there is no large planet to provide the pull of gravity. However, as students can see, the astronauts still exist, so they still have their original mass.
Varying Weight Not Mass
A quick and easy demonstration is to have a student (or a teacher) stand on an electronic scale. The scale will show the person's weight. The teacher can then have students place their hands on the person's shoulders and push down. The apparent weight on the scale of the individual will increase. Students can see from this demonstration that a person's weight (the force on their body) can change, even though their mass stays the same.