A compound is a substance formed when two or more different chemical elements are chemically bonded together in a fixed proportion. Milk, Tea, and Coffee are all mixtures. They consist of various substances physically mixed together, not chemically bonded in fixed ratios. "Sugar" typically refers to sucrose (C₁₂H₂₂O₁₁), which is a molecule composed of atoms of carbon, hydrogen, and oxygen chemically bonded in a fixed ratio, making it a compound.