One that is used on the Medstone machine and on the majority of machines is the spark gap. Just like the Eurolith machine uses an electric spark, we use a spark plug. If you look at what's generated, it looks very much like an automobile spark plug, it's about 5-times the size of it, a little bigger, it's a spark plug, and a spark is discharged over that gap and there's an explosion, basically, underwater that's caused by that and it's the shock wave that comes from that explosion that we're using to break up the stone. You want to make sure that from wherever the shock wave was generated that there's no difference in acoustical interface between where the shock wave was generated and where it's going, and so you can use water. The original shock wave lithotripter was in a big tub of water. We had this big power lift hydraulic chair. We put the patient in and drop them down into this huge tank of water, and so that was probably the best coupling because there was no possible loss of power across any interfaces. It's been found that if you just use acoustic gel, and in our case with the Medstone machine, mineral oil, there's very little loss of the power and of the shock wave, and so you really don't need to be dumping patients in tubs of water, and so we now have what we call dry lithotripters or the tabletop models that we use today.