Question

why does a balloon bursts if kept in sun?

Answer 1

due to Sun's heat, the kinetic energy of gaseous particles inside the balloons also gets increased and the balloon expands and burst 

Answer 2

balloon bursts , when it is kept in sun light or in a hot place due to the expansion of gaseous parties in side it. this causes the more pressure inside the balloon, this leads to the burst of the balloon.
hope it helps u. ...