Combine

Some things can be combined to gain more mass, for example Wood scrap, Iron Lumps, 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.