Nala and Nila were the divine monkeys who built the floating bridge of Ramayana. They were able to float stones and huge boulders on the ocean due to a curse by Sage Sutikshna.
Sage Sutikshna used to worship two Shaligram stones in his ashram. One they the two Shaligram stones went missing. The other saints and residents of the ashram had no idea as to how the Shaligram stones disappeared from the ashram.
Sage Sutikshna could not do anything since the Salagrama stones went missing. He could not pluck fruits, get edible roots, or gather flowers. Days passed thus. The summer arrived. The lake near the ashram went dry. Water had to be drawn from the well like pit in the middle of the lake. One morning when Sutikshna tied a rope to a vessel and drew water from the pit, he felt something heavy. He gradually took up the vessel of water and examined it. And lo! There were his missing Salagrama stones. His happiness knew no bounds when he saw them. He took the Salagrama stones to the ashrama and offered worship as usual.
One day, the sound of vessels moving about came from the worship room in the ashram. Sutikshna rushed to the room and saw that his Salagrama stones were being carried by two monkeys, each one taking a Salagrama stone on its head. Sutikshna ran after them. The monkeys threw the Salagrama stones into the lake and fled. Later, Sutikshna somehow managed to extract the Salagrama stones from the lake.
This happened several times and eventually Sutikshna got enraged. At the same time, he felt it was pointless to curse the monkeys. After some thought, he cursed those two monkeys thus: ‘From now on, whatever you throw on water would float.’ Since then, the monkeys regularly threw his Salagrama stones into the lake and Sutikshna brought them back to the ashrama.
Those monkeys that were thus cursed by Sage Sutikshna are well known to us. They are the monkeys Nala and Nila occurring in the Ramayana. When Hanuman gave the idea of constructing a bridge across the ocean, Sri Rama ordered that only Nala and Nila should arrange the stones for the bridge. That is why even big boulders did not sink and they floated on the ocean. Thus, Sage Sutikshna’s curse had a good ending.
Source - Prabuddha Bharata December 2020 Issue.