申論題

13, A little boy, named Johnny, got a beautiful red balloon at his friend's birthday party on a snowy day. The balloon was filled with helium up to 0.25 MPa. After having a good time indoors, Johnny and his father decided to walk home in the -5.0  ℃ weather. As they walked, Johnny's balloon shriveled up and flopped to the ground. It seems that the balloon cooled down and shrank.

i) Why did Johnny's balloon shrink? Use the ideal gas law to explain this phenomenon.