Some things can be combined to gain more mass, for example [[Wood scrap]], [[Iron LumpsLump]]s, [[Rye]], and others.
To [[Combine]]: Activate the first item (double click when it's in your inventory), then right click the second (identical) item and select [[Combine]].
You can also combine more than two items at once. For this to work, activate the last item in a pile of identical things, then right click the first entry (the one with a -/+ sign) and select combine.