Theory:

Leaf after leaf of the fine gold the swallow picked off, till the Happy Prince looked quite dull and grey. Leaf after leaf of the fine gold he brought to the poor, and the children’s faces grew rosier, and they laughed and played in the street. “We have bread now!” they cried.
 
Then the snow came, and after the snow came the frost. The streets looked as if they were made of silver. Everybody went about in furs, and the little boys wore scarlet caps and skated on the ice.

The poor little swallow grew colder and colder, but he would not leave the Prince, he loved him too well. He picked up crumbs outside the baker’s door when the baker was not looking, and tried to keep himself warm by flapping his wings.

But at last he knew that he was going to die. He had just enough strength to fly up to the Prince’s shoulder once more. “Goodbye, dear Prince!” he murmured. “Will you let me kiss your hand?
 
“I am glad that you are going to Egypt at last, little Swallow,” said the Prince. “You have stayed too long here but you must kiss me on the lips, for I love you.”

“It is not to Egypt that I am going,” said the swallow. “I am going to the House of Death. Death is the brother of Sleep, is he not?”

And he kissed the Happy Prince on the lips, and fell down dead at his feet.
Explanation:
 
Slowly, the bird took out the thin gold layers from the statue of Happy Prince, and as he removed the layers of the gold, the Happy Prince’s statue started looking dull and grey. As the statue of Happy Prince started becoming dull and grey, the children grew happier because they were getting food to eat through it.
 
Later, the place started snowing, and when a lot of snow fell, everything began to freeze. As it was a colder season, everyone was wearing clothes made of fur. In addition, small children were wearing red coloured caps and were roaming here and there. They were skating on the ice. Then the bird felt very cold, but he kept on sitting near the Happy Prince’s statue.
 
The swallow did not like to leave the Happy Prince because he loved the Prince. The swallow goes to the bakery (as a daily routine), where the baker bakes the bread. He picked up the bread crumbs from outside the shop when the baker was not noticing him. Then he tried to keep himself warm by flapping his wings. Later, the swallow realised that he was going to die because of the cold weather. Also, he thought that he had only enough strength to fly back to the statue of Happy Prince.
 
Then the swallow came near to the Happy Prince to say Goodbye. Meanwhile, the swallow asked the Prince, “Will you let me kiss your hand?" After hearing that, the Prince said that he was happy that the swallow was going to Egypt. Then Happy Prince said to the swallow that it had stayed with him for a very long time, and now, it was his time to go to Egypt. Happy Prince wanted the bird to kiss its lips instead of the hands because the statue loved the bird. The swallow said to the Happy Prince that he was not going to Egypt; instead, he was going to the house of death. Here the bird meant “House of Death”, which means he was going to die. Also, he said that he was going to meet the "brother of Sleep". At last, the swallow kissed the Happy Prince on his lips and fell at the feet of Happy Prince and died.
 
Meanings of the difficult words:
 
S.No
Words
Meanings
1
Dull Lacking interest or excitement
2
FrostA deposit of small white ice crystals formed on the ground or other surfaces when the temperature falls below freezing
3
Snow Atmospheric water vapour frozen into ice crystals and falling in light white flakes or lying on the ground as a white layer
4
Skate A special boot with a thin metal bar attached to the bottom that you wear to move quickly on ice, or a boot with four small wheels fixed to the bottom so that you can move over a hard surface
5
ScarletOf a brilliant red colour
6
FeetThe lower extremity of the leg below the ankle, on which a person stands or walks
Reference:
National Council of Educational Research and Training (2006). Moments. The Happy Prince – Oscar Wilde (pp. 28-35). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.