Hydrogels of the so-called smart polymers or environment-sensitive polymers are important modern biomaterials. Herein, we describe a hands-on activity to explore the pH-responsive characteristics of hydrogels using a commercially available ionic soft contact lens that is a hydrogel of poly(2-hydroxyethyl methacrylate-"co"-methacrylic acid), poly(HEMA-co-MAA). The observed oscillatory swelling and contracting behaviors of the ionic soft contact lens in response to the changes in solution pH exhibit the reversible pH-responsive property of the hydrogel. Chemical structures of functional groups and the associated intermolecular forces in connection with the physical states of the hydrogel are demonstrated. Solutions of household materials such as vinegar, baking soda, and table salt were used to provide solutions with various conditions. This activity covers the chemistry of polymers and the topics relevant for general chemistry such as molecular interactions, acid-base reaction, solution concentration, and osmosis. The swelling and contracting of the ionic soft contact lens in solutions with various tonicities also make a unique class demonstration for teaching the topic of osmosis due to the quick response of the lens to the environmental solutions.