Gravity Vs Balloon

Classical Mechanics Level 1

It is well known that a balloon filled with helium flies upwards, against the force of gravity.

Why does a balloon fly against gravity?

Helium gas molecules are much smaller and lighter than oxygen and nitrogen molecules. The helium gas inside a balloon is under less pressure than atmospheric pressure. Gravity pushes upward on the balloon. The helium gas inside is less dense than the air around the balloon.