Results!
SUGAR: Stir and watch it disappear! The water is clear. Sugar DISSOLVED completely. The water tastes sweet (the sugar is still there, just invisible).
SALT: Just like sugar, salt dissolves completely! The water is clear. It tastes salty. Salt DISSOLVED.
SAND: Stir all you want, the sand sinks right back to the bottom. Sand does NOT dissolve. It is too hard for water to break apart.
FLOUR: This one is tricky! When you stir, the water gets cloudy and white. It looks mixed, but wait five minutes and the flour starts to settle to the bottom. Flour does not truly dissolve; it just gets suspended temporarily.
OIL: Oil floats on top and does not mix with water at all. No matter how much you stir, it separates back into a layer on top. Oil and water do NOT mix!