Put a fresh egg in a glass of vinegar. Leave the egg in the vinegar until the hard crust dissolves and remains only an elastic membrane. Put the egg thus obtained in water. In 3-7 days, the egg will increase its volume.
The experiment is even more fun if we put the egg from vinegar into a bottle with water and leave it in the bottle to grow. No one will understand how we introduced a large egg into a bottle without breaking it. Besides, in the bottle it is not visible that the egg has no thick shell, and the glass behaves like a magnifying glass, so the egg seems even more bigger than in reality.
To decrease its volume, put it in spirits or salty water. The water inside the egg comes out very quickly through the elastic membrane and the egg will decrease its volume.
The explanation is that the water behaves like a solvent and, spontaneously, it moves to places where the concentration is higher. The phenomenon is called osmosis, based on the exchanges between two liquid solutions of different concentrations in liquid phases slightly separated by a wall. Osmosis is a phenomenon of diffusion of the matter demonstrated when molecules of solvent pass through a semi-permeable membrane separating two solutions whose solute concentrations are different; the solvent is then transferred from the less concentrated solution (hypotonic medium) to the most concentrated solution (hypertonic medium) to equilibrium (isotonic media). This notion is particularly useful in physiology and cell biology to explain the chemical exchanges within living organisms. A hydrostatic pressure difference between the two liquids causes the solvent to move in the opposite direction until the osmotic pressure is as high as the hydrostatic pressure. This is the reverse osmosis phenomenon.