After a workout, the body feels hot due to the exothermic reactions involved in cellular respiration, where glucose is oxidized to produce energy, water, and carbon dioxide. During aerobic exercise, oxygen is utilized efficiently to break down glucose, while anaerobic exercise leads to lactic acid production due to insufficient oxygen. This increase in metabolic activity generates heat, similar to how a car engine requires cooling. The body's temperature rises as it works to dissipate this heat through mechanisms like sweating. Understanding these processes highlights the physiological responses to exercise and energy production.