Ha ha yeah sorry it's a bit of a mess!
I've got a sheet of glass laid down flat. I've got a suction pad with flexible arm attached to the glass. The arm holds a cardboard tube vertically. The tube is there to allow me to drop the pebbles in exactly the same place each time.
I use a drinking straw with some washing up liquid solution to make a bubble on the glass, right under the cardboard tube. I drop a pebble down the tube, and it lands on the bubble.
I've made a laser trigger system (the guts of it are in the 2nd photo) which basically opens the camera shutter, then waits for the laser beam to be broken. When the beam is broken it fires the flashes (2 x 430EXII) and closes the shutter. I've set the beam up just under the carboard tube so when the pebble falls out of the tube it breaks the laser beam. I've set a 12ms delay between the beam being broken and the flashes firing.
The coloured background is produced by a load of coloured elastic bands hanging on a bit of string behind the bubble....
I need to have a tidy up actually - might take another clearer set up shot then.
Thanks a lot for the compliments!!