A gun with it explosive energy can accelerate the bullet ball, with mass m, from zero velocity to velocity Δv. After the gun shoots a ball into the wall, the wall receives an energy of 1/2 m(Δv)2 which is converted from the explosive energy to the kinetic energy of the ball and then to heat and/or deformation energy of the wall. When a person carries the same gun on board of a high speed train in the speed V, the observer on the earth will see the kinetic energy change as 1/2m(V+Δv)2-1/2mV2 = 1/2m(Δv)2 + mVΔv when the gun on the train shoots the bullet ball. Where does the extra energy mVΔv come from? Is the gun powder explosive energy a frame-of-reference dependent quantity? Does it concern the theory of relativity by Albert Einstein? In this paper, the extra energy mVΔv is defined as the mutual energy of the relative motion which is a frame-of-reference dependent energy and may be transferred from one reference frame to the other.