The lava lamp is a groovy enough device, but it is based upon rather simple principles. Two liquids are suspended in a glass reservoir; each liquid is of a different color and viscosity. The lighter liquid is most likely transparent, while the heavier, possibly waxy liquid is opaque, and ideally of a darker hue. The metal base of the contraption holds a light bulb, which not only illuminates the glass reservoir (and, hence, a corner of the room) but also heats the liquids.
Now, here's the tricky part: when the thicker liquid starts to warm up, it starts to rise to the top of the glass container - but since it heats unevenly, it does so in individual globs, which are nice and round and smooth-looking. Once at the top, these globules start to cool down, and after a while will start to sink again; meanwhile other globules are starting to rise, with a lot of the mess connected together in long, elongated columns of the stuff. The visual effect is one of a constantly oozing, undulating mass, suspended in the background liquid color. It is soothing and pleasing to look at - and, under the proper medicinal frame of mind, rather trippy.
The original lava lamp was marketed as the Astrolight, and was invented in 1964 by Craven Walker, and Englishman. Rights to the headtrip-inducing contraption were purchased by a Chicago businessman, who no doubt had his finger on the pulse of the youth market, because by 1970 over two million of the darn things had been grabbed up by consumers. By the 1980's, the things were considered tragically unhip, but by that time a total of about seven million had been sold.
Along with the smiley face, the Nehru jacket, and flower tattoos, lava lamps were very much a product of a specific period in American culture. Though that time may be in the ever-receding past, such things still carry a certain novelty and nostalgic quality to those who are inclined to enjoy such unfashionable items.
Besides, they're still, y'know, really fuckin' cool.