First I used three BBQ skewers, a drinking straw, a cotton reel, two empty pots of apple sauce, four old popsicle sticks, some string, a coffee stirrer, a little bit of styrofoam, a cardboard box, four plastic beads, and a few wooden beads that were lurking around to make a table top weighing scales that the kids could put together and take apart themselves. Here are the parts that I made for the kids to fit together.
The poles go through the cardboard box, with the beads being underneath to hold it steady. Then the two straws with the bamboo skewer glued into them are threaded through the center of the cotton reel. Then the two straws are fitted over the two poles, so that the contraption that is glued around the cotton reel pivots on the BBQ skewer. Then the pots are hung on either side (they are held on by hooking the string over the green plastic beads). I could have made it more sturdy, but I wanted the kids to be able to build it, take it apart, store it flat and build it again later.
We just used what we had handy, but there must be a bazillion different ways to make a simple device that you can compare the weight of two things with. The little red coffee stirrer with the styrofoam arrow made it easier for the kids to see which was heavier when it was a close call between the two weights.
I provided the kids with a bunch of different objects to try weighing, some small and heavy like coins, some larger and lighter like corks. It wasn't long before they went off to find other things they could put in the scales as well, like small plastic princesses and toy animals.
Once they had played with that one for a bit and we'd tried a few different activities with it, we set about making a larger scales that could hang in the doorway. This one was even easier to make. All we used was a plastic coathanger that had hooks on either end, a wooden bead as a weight on a string to always mark straight down. Then I glued on a bamboo skewer with an arrow at right angles to the bottom of the coathanger, so that it would point to either side as the coathanger pivoted. We hung two cardboard strawberry punnets on the hooks at either end of the hanger.
The kids played filling the buckets with various stuff to see which was heavier.
Later that night I figured why not make it into a proper game, so I got out some larger matching buckets to hang on it and drew a little dial that could be threaded over the weighted string that the wooden bead was hanging from. I had to cut the bamboo pointer a bit shorter to read the dial. It wasn't going to be accurate to read for real weight measuring units because the dial was too low for where the scales were pivoting from, but the kids were able to see who's bucket was heaviest and how many "points" they had won. Notice that because the dial is below the pivot point the pointer points to the lighter side, hence colouring the sides to match the buckets, so the kids could see who had won more easily.
If I was doing this again (and we probably will) then I'd loose the wooden bead thread and instead thread a dial onto the string that the hanger was on, so the center of the dial is where the hanger pivots from and have the pointer pointing upwards. That way the pointer would point to the heavier side and also if you had proper weights then you could calibrate the dial (with older kids). Kinda like this...
Anyhoo, they still had a tonne of fun with it and discovered that wooden blocks are infact heavier per unit volume than barbies. Barbie is not as dense as wood! I still don't know whether she is as thick as two short planks though (have I just confused all the Americans reading with that one?)