Interesting fact: A hot object really does weigh slightly more than the same object when cooler, although the difference is tiny beyond everyday measurement.
The answer: Heat is energy, so it has mass equivalent through E = mc^2. For example, adding about 4,186 joules of heat to 1 kilogram of water raises it by 1 degree Celsius and adds only around 4.7 x 10^-14 kilograms of equivalent mass.
Heat cannot be weighed like a brick because it is not a separate material. Instead, physicists talk about how thermal energy changes the total mass-energy of a system. The effect is tiny in everyday life, but it is conceptually important because it connects thermodynamics with relativity and explains why hotter systems technically contain slightly more mass-energy than cooler ones.
