What is one thing which is 2kg if dry, 1kg if wet and 3kg if burnt?

Answer is Sulphur because Sulphur has a specific gravity of 2. That means if you were to weigh a 2kg lump of Sulphur under water (i.e. WET) it would have an apparent weight of only 1kg. All things weigh more when they burn (oxygen gets added). Sulphur has an atomic number of 16. If it combines with oxygen (atomic number 8) on a one-to-one basis, it will weigh 1 1/2 times as much, That means 2kg will weigh 3kg.