Jessica took a Mylar balloon outside on a very hot day. As she waited for the bus the balloon began to get bigger. What is the best explanation of this?

The hot air outside the balloon was transferring heat to the gas inside the balloon. Because the gas could not escape because the balloon is a Mylar balloon, the gas expanded, making the balloon larger.

Mylar balloons are different from usual latex balloons. Mylar balloons are made of Nylon with metallic coating. The latex balloons are porous so they do not stay inflated for a very long time while Mylar balloons are not porous so they can stay inflated for a longer time.

When Jessica took the balloon outside the gas inside the balloon started expanding. But the expansion was due to the expansion of the outer layer of the balloon itself. As metal expands due to heat, the balloon got bigger.