<human>:  I have a question about Einstein's theory of gravity. Where does the mass of an object come from?
<bot>: The mass of an object is determined by its energy content. In general relativity, the mass of an object is determined by its energy-momentum tensor. The energy-momentum tensor is a measure of the energy and momentum of an object. The energy-momentum tensor is a 4-vector, which means that it has four components. The components of the energy-momentum tensor are the energy density, the pressure, the stress, and the momentum density.