Molecules are moving around, rapidly, and randomly all the time, and this is called Brownian motion. It was named after botanist Robert Brown (1773-1858), who discovered random-jerky motion by observing pollen grains suspended in water. Even though water was completely still, pollen grains were still moving. However, he didn't found out why, assuming that it was evidence of life. It was Einstein that analysed theoretical Brownian motion.
In liquid/gas state, because spaces between atoms are empty, motion of molecules are quite rapid, and random. But in solid, because atoms are compacted quite tightly and fixed at one point to form solid, this motion is restricted to vibration about their equilibrium positions.
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