Two potential ideals:
Absorb mass by sucking in:
In a sand desert environment you could imagine an animal that sucks sand into a body cavity / 'sand bladder'
This was already suggested by another answer. (Dry) sand has the advantage, that it can quickly be sucked up.
Absorb Mass by chemical reactions:
Looking at @FluxFlowFreq answer:
Any oxydation, that doesn't expel exhaust gases will gain you mass
There are exothermic (e.g. burning hydrogen) and endothermic (e.g. burning Nitrogen) oxydations. With a right mix you might avoid abrupt temperature changes.
With oxydation the mass would be gained from the surrounding air (O2 or if you find a nice chemical reaction even the N2)
If the creature could store considerable amounts of hydrogen (perhaps chemically bound) it could burn this hydrogen into water (steam if not being cooled down quickly) If that water is kept the creatures mass would increase. (Two H2 molecules can bind one O2 molecule) It would be that oxygen, from the air that is bound and attributing to the mass.
This would mean, that a lot of gas surrounding the creature would disappear quickly, resulting in a bang, or an whirlwind or similiar.
Generally speaking if a creature can provoke a chemical reaction that absorbs molecules from the air Oxygen is the one, that springs first into mind, but perhaps Nitrogen could also be absorbed especially with the energy produced by transforming H2 and O2 into water, then the mass could be increased.
Problem is of course how to not burn die / because of overheating.
You had to find chemical reactions, that are not too exothermic or that are compensated by parallel endothermic reactions, that don't release mass.
The tricky parts are probably:
- the energy balance (not too much overheating / freezing down). You'd need a mix of exothermic and endothermic reactions
- controlling the speed of the reaction
- ensuring, that all the 'burnt' material is not expelled. but kept in a bladder, body cavity, or being attached to the body
- ensure, that the gained mass is big enough to make a difference.
Burning steel wool for example is also a chemical reaction that gains
In fact any oxydation, that does not expell exhaust gases like CO2 or CO will gain mass.