Why does heating a sample increase its mass?

When the body is heated, the kinetic energy increases. What does it mean? Once the kinetic energy has increased, it means that the speed of propagation of molecules has also increased. In addition to speed, mass increases, which is in direct proportion to energy. Its increase is not significant, it can be seen only with very accurate measurements. In ordinary life, with an increase in temperature, changes are not noticeable at all. Since the increase in the speed of molecules compensates for the high speed of light.