The Night Watchman's Dilemma

The Story: Rajan has worked the night shift at a textile warehouse in Surat for eleven years. He knows every corner of the building, every creak of the floorboards, every shadow. One Tuesday evening, just as he settles into his chair with a thermos of chai, the power cuts out. The entire building goes dark. Rajan reaches into his drawer and pulls out a torch — but the batteries are dead. He needs to go downstairs to reset the circuit breaker, which means passing through the storage room. Inside that room are two large barrels. One barrel is filled with engine oil. The other is filled with drinking water. Both barrels look identical in the dark — same size, same shape, same material. Rajan knows that earlier in the week, a worker spilled some drinking water near the water barrel and some engine oil near the oil barrel. The floor around each barrel still carries the faint smell of whatever is inside. He kneels down and sniffs near one barrel. It smells faintly of water. The Challenge: Without any light, without opening the barrels, and without tasting or pouring anything — how can Rajan confirm with complete certainty which barrel contains water and which contains oil?