One day a deer was rubbing itself against a sage's fire- kindling mortar. and as it turned to go it got entangled in its horns.
Alas! The deer is running with my fire kindler.
Can you get some water for us
I can see water plants and cranes there
O son of Madri! answer my question and then drink the water
The sage shouted and rushed to the Pandavas for help in his distress
Ssurprised that Nakula didn't return, Yudhishthira sent sahadeva to see whats the matter.
The voice was heard again.
Nakula went to get the water.
O Sahadeva,this is my pool.Answer my questions and only then may you quench your thirst
Puzzled and worried that Sahadeva also did not return, Yudhishthira sent Arjun to see whether the brothers had met with any danger.
Nakula was surprised but carried his intense thirst and heedless of the warning, he drank water. At once overcome by great drowsiness, he fell down to all appearance dead
And bring water.
As Sahadeva was thirsty too so first he rushed irresistiblyto the water to quench his burning thirst.
Like Nakula, Sahadeva also did not heed the warning and drank the water and at once dropped down.