As per normal human thinking it is impossible to build a bridge on sea.
Then how could Nala, a mere vanara, build the Rama Setu across the sea?
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In Ramayana, almost all Vanaras are the incarnation or Ansha of some deity and so was the Nala. Nala was the son of Vishvakarma (the divine architect) and he had a boon of his father. It is described in Yuddha Kand, Chapter 22 as:
अयम् सौम्य नलो नाम तनुजो विश्व कर्मणः | पित्रा दत्त वरः श्रीमान् प्रतिमो विश्व कर्मणः || २-२२-४४
O, excellent man! This one, named Nala, a glorious person, is the son of Vishvakarma; who was given a boon by his father and is equal to Visvakarma.
एष सेतुम् महाउत्साहः करोतु मयि वानरः | तम् अहम् धारयिष्यामि तथा हि एष यथा पिता || २-२२-४५
Let this greatly energetic monkey build a bridge across me. I can hold that bridge. He is just the same as his father.