Materials
Scales
Masses
Objects to weight, such as sticks, small pebbles, larger rocks, plastic dinosaurs (forr the little kids who can't add large numbers, toys work well), a potato, etc.
Worksheet for masses (below)

Procedure

Background
Explain the concept of mass (weight, although different than mass, is probably easier to explain to the younger grades).  Demonstrate how to use whatever scales are available.  Generally, I get a balance-style scale and explain that it works like a teeter-totter (for the K-2nd graders).  Go ahead and ensure that all the scales are balanced properly.

Why do need masses that are in the same units?
Have each of the groups pick up a bunch of small pebbles and a stick.  Place the stick in one side of the scale.  Next, have them measure the stick in terms of pebbles.  For example, one stick may weight 14 pebbles.  Then, have them exchange the stick with another group and repeat the process.  The students will realize that the stick that weighted 14 pebbles for them may only weigh 11 for the other group.  Now, repeat the process using the masses.  Both groups should get the same (or very close to the same) answer.  The reason is that the pebbles are not uniform in size or density.  Therefore, each of the pebbles has a different mass and will give different readings on the scale when weighed.  The experiment may then be repeated with the other materials.  Often, it is a good idea to create a sheet to record the results and give the students practice using the scales.
 

WORKSHEET

                                                  Weight in pebbles                   Weight in grams
1.  Stick 1

2.  Stick 2

3.  Small rock

4.  Large rock

5.  Potato

6.  Pencil

7.  Cup with water

8.  Empty cup

9.  Weight of water
(difference between 7 and 8)
 
 

Shawn Len, 2000.
 
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