Take two gas jars.
In one gas jar fit a bromine candle and in the other jar air will be present.
When you open the bromine candle all the molecules of bromine by the process of diffusion move from higher concentration to lower concentration. This shows that particles are constantly in motion. When you fit the bromine vapor filled can dle with the other gas jar, we can see that going against gravity the bromine vapors get uniformly spread throughout the two gas jars. This also shows that bromine and air molecules are constantly in motion.