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Ok, so you know how I said molecules bounce off walls? Not actually true! 😬 I lied to keep it simple, but I think you're ready for the truth: It turns out that any time a the only way wall-molecule interaction plays out. most of the time a gas time any gas molecule hits a surface it actually sticks to that surface, just if only temporarily, like how your breath fogs the a mirror; this called adsorption (not to be confused with absorption).
And also just like fog on a mirror vanishes over time, those stuck molecules will eventually jump back off of the surface (i.e. evaporate, outgas, desorb), flying of off in a random direction just as fast as it was when it hit the surface..
How long a molecule stays stuck before desorbing from a surface depends on the type of molecule, the temperature, and the what the surface is made of. Evaporation of a molecule can happen any time between almost instantly and basically never. And we'll never be a able to predict the moment exactly, but we usually have a general idea for how long, based on things like temperature, pressure, and material.
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