Q.How do you define energy?

The internal energy $U$ of a thermodynamic system is the sum of the kinetic and potential energies of all the molecules in the system, measured with respect to the centre of mass of the system: $U = E_K + E_P$, where $E_K$ (internal kinetic energy) comes from translational, rotational and vibrational molecular motion, and $E_P$ (internal potential energy, e.g. bond energy) comes from molecular interactions. For an ideal gas, molecules are assumed not to interact, so $U$ consists purely of kinetic energy, depending only on temperature and the number of particles, and not on volume; this is not true for real gases (e.g. van der Waals gases), where intermolecular interactions contribute to $E_P$. Internal energy is a state variable: it depends only on the system's initial and final states, never on the path taken to get there. Crucially, internal energy is associated only with the random molecular motion and the potential energy from molecular interactions -- the bulk kinetic energy of a moving container of gas, or its bulk gravitational potential energy, are never part of internal energy. Internal energy is an extensive variable, depending on the mass/size of the system, so a larger volume of lukewarm water can have more internal energy than a smaller volume of much hotter water, even though heat still flows from the hotter to the cooler sample, independent of which one has the greater internal energy.