Why does water bead up on a waxed car but spread out on clean glass?

Water beads on a waxed car because wax is a hydrophobic, or water-hating, surface that causes droplets to contract to minimize contact^[3]. Conversely, water spreads on clean glass because it is hydrophilic, or water-loving, meaning adhesive forces between the water and glass pull the liquid outward^[3]. This behavior is determined by the contact angle: angles over 90 degrees indicate hydrophobic surfaces, while angles under 90 degrees signify wetting^[3].

For practical applications, adding wetting agents to detergents lowers surface tension to help liquids penetrate surfaces^[3]. Similarly, hydrophobic coatings protect fabrics and improve visibility on windshields by forcing water to bead and roll away^[3].