The Frog Rock is a natural sandstone boulder, mounted on a short steel pedestal. It seems to float about two inches above the floor. The stone has been hollowed out, and inside is a small, battery operated microprocessor. This processor spends most of its time asleep, but periodically wakes up to reproduce the digitally recorded sounds of insects and amphibians. During the daytime (a light sensor enables it to make this distinction) the Frog Rock periodically reproduces the call of a Pacific Tree Frog. After dark, the Frog Rock sometimes reproduces the call of a pond frog, and sometimes reproduces the calls of field crickets. Low power CMOS circuitry allows it to play for many months on a set of batteries.
The point of this is to bring a little wild, outdoor ambiance into the living room. The digitization is processed at 22 KHz, so the sound samples are convincing (much more so than inexpensive manufactured novelties you may have heard). Guests are invariably fooled into searching for the creatures.