Yes, particles in a liquid are constantly moving. This motion is called Brownian motion, named after the Scottish botanist Robert Brown who first observed it in 1827. Brownian motion is a random motion of particles suspended in a fluid (liquid or gas). It is caused by the collision of the fluid molecules with the suspended particles. The smaller the particle, the faster it moves. Brownian motion is also affected by the temperature of the fluid: the higher the temperature, the faster the particles move.