Heavy 5 Gallon Water Bottles
According to OSHA.gov:
Potential Hazard: Loading bottled water onto water coolers exposes employees to several risks. A bottle weighs approximately 48 pounds. To load a bottle, the employee must lift it up, over and onto the water cooler (while trying to avoid spilling the bottle). This is an awkward task that places strain on the employee's back, shoulders, arms and legs. Another hazard is that employees may slip and fall as a result of spilled water.
Link To OSHA Water Delivery Reports