The safety staff at a large chemical works became concerned at the number of “slipping and falling” accidents which were occurring on a particular staircase. The stairs were cleaned, then resurfaced, then removed and replaced with a new set—before the real cause of the accidents became apparent. At the top of the stairs was a tank for storing a certain chemical. It was fitted with a manually operated valve which allowed its contents to be fed down a pipe into a second tank on the floor below. This lower tank was fitted with an indicator gauge to show how full it was. To run a measured quantity of the chemical into the lower tank, somebody had to climb the stairs to open the valve; then run downstairs to check the indicator; then run up the stairs again to close the valve.