Which thing that dry 2 kg wet 1 kg after burn 3 kg?

The riddle refers to fire. When you burn a substance, such as wood, it typically weighs more before burning due to moisture (wet weight) and less afterward due to the loss of mass as gases and ash are produced (dry weight). In this case, the "2 kg wet" refers to the weight of the wood before burning, the "1 kg" represents the moisture content, and "after burn 3 kg" describes the total weight of the gases and ash released during combustion. Thus, the riddle plays with the concepts of weight before and after the burning process.