Question

how does the heat gets generated when people jump up and down on a cold day?

Answer 1

we tend to sweat as heat is produced in our body during physical exercise

Answer 2

Your body stores energy in the form of fat. When some activity (like using muscles) demands more energy, it begins to convert some of that fat to sugars carried by the bloodstream to where it's needed. The muscles use that energy to produce the motion you're asking them to make, but a byproduct is heat. To get those sugars delivered requires more blood flow, helping to bring more heat to your extremities. To conserve energy, your body tends to let them cool down first, so it can be sure to keep your vital organs warm. All these work together to raise the overall metabolic activity level of your body, which means more stored energy is converted to heat.