I've seen them all over and I had one years ago, who didn't have a lava lamp ?:)
I saw a tutorial on how to make a lava lamp a while ago, and I thought it would be a great opportunity to show the boys the magic of science. I followed this tutorial if you want to try it at home. We didn't use the bottles from the tutorial, but used a couple of the canning jars I already had.
The boys LOVED it. They thought it was super neat to see the bubbles reacting and to see the color flow all around the jars.