The concept is pretty simple - it's a baking soda volcano, but you have the kids build the volcano out of LEGO, around a styrofoam cup. Then, you can let it erupt all over the LEGO, cleaning them in the process. A drop of dish soap makes it more foamy, and a couple drops of food coloring make it even more exciting.
What I DidI had set up little trays (we had them in the supply closet), each with one styrofoam cup and a couple handfuls of LEGO. The directions were: Build up the LEGO around the cup, as much as you want, as long as it still fits inside the tray. (For some reason, this was really hard for some of them to understand, but they got it eventually.)
When each kid was done building, I came over and put three spoonfuls (which is WAY TOO MUCH, see below) of baking soda into the styrofoam. Then, the kid came back to my little side table and got a plastic cup, which we filled halfway with vinegar, and added a couple drops of dish soap and a couple drops of whatever color food coloring they wanted.They stirred it with a popsicle stick, poured it into the baking soda, and watched in amazement as their volcano erupted.
This whole project went fairly quickly - one got bored and wanted to see the eruption, and then once we saw one eruption, everyone had to do their own RIGHT THEN. I even had a few kids come in who were way too cool for Science Club, until they heard there were eruptions, and then they had to come do their own. A couple kids did two volcanoes each, which was fine because I had the supplies.