When comparing objects that float or sink in a particular liquid, we can determine something about the relative density of the objects and/or the liquid.  The density of water is approximately 1 g/mL.  Any object that floats in water will have a density less than water and the density of an object that sinks in water will have a density greater than that of water. 

We could further narrow down the range of possible density values of the blocks by placing them in different liquids to see if they floated or sank.    For example, ethanol has a density of 0.789 g/ml.  If the block that sank in water (i.e. density less than 1) floated in ethanol, we would know that the density of the wood block is between 0.789 g/mL and 1 g/mL.