Interesting fact: In modern physics, even a charged battery technically weighs slightly more than a discharged one because stored energy adds mass.
The answer: Energy has mass equivalent according to E = mc^2. One joule corresponds to about 1.11 x 10^-17 kilograms, an incredibly small amount that is usually far below what ordinary scales can detect.
The connection between energy and mass became clear with relativity. Energy is difficult to weigh directly because its mass equivalent is tiny in everyday situations, but the principle is real and measurable in high-precision physics. It matters because it links batteries, heat, light, nuclear reactions, and gravity under the same physical framework.
