system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do balloons deflate over time?
gpt
Gas molecules are small and rubber, particularly thin rubber like that used for balloons, is very slightly porous. Because the pressure inside is greater than the outside air, molecules will diffuse through.The rate of diffusion is governed by 

1) the size of the molecule so something like helium which is just single atoms and therefore tiny will escape quicker than molecules like nitrogen consisting of two atoms per molecule.

2) the porosity of the skin so a Mylar balloon, and particularly one with a metal coating, will deflate more slowly.