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When Can Two Entities of Matter Occupy the Same Space ?

When can two objects, or entities of matter, occupy the same space ?  How can this be true ?

Take for example, a half-inch sugar cube and an equal volume of water.  When these two attempt to occupy the same space, the sugar dissolves in the water, and a new mixture is formed. 

When two entities of matter mix with each other they will appear to occupy the same space - but not without altering each other’s original state.  The sugar cube is no longer what it used to be, nor is the volume of water.  The sugar cube is dissolved in the water (melted) and the volume of water is now saturated with sugar.  The molecules of each have interspersed to form sugary water.  This phenomenon constitutes a mixture, as defined in chemistry. 

Likewise, if air mixes with water droplets, it forms a new mixture - steam.  Humid air is not the same as dry air, and if an airplane propeller is designed to push dry air, it will not work the same in humid air.  The propeller will not be pushing air, but a combination of air and water. The humid air introduces a hydrodynamic drag onto the propeller and can slow it down.  Since things in general move quite slower in water, the forward velocity of an aircraft can be substantially decreased in humid air (or air that is saturated with rain).