Due to a water shortage on the desert set, Steven urged all the actors to begin saving their urine. Well, the next day the water shipment FINALLY came in. Still, Steven didn't want to let a perfectly good cup of pee go to waste, soooo...
-- OR--
Steven demonstrates to Harrison Ford the finer points of childish pranks.
-- OR --
Putting that leftover Bantha spit to good use...
-- OR --
Steven demonstrates how to properly flaunt wealth by wasting a precious resource... water in the middle of the desert.
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(BTW, after seeing Steve and Harrison's reaction to this let's-pour-water-on-Harrison-Ford-teehee-teehee prank, I exclaimed out loud, "My God-- they really ARE nerds!")