I've just started getting interested in physics and I am only a few (basic) books deep but there is something that has been distracting me that I can't come to a logical conclusion about. I assume this means there is a flaw in my basic understanding and wish to fix it asap. Here goes. I am assuming the following statements to be true (as is my understanding which, as I've said, is likely flawed). *The kinetic energy of an object is greater the faster it is moving. *As motion is relative, Person A on the ground can observe Person B whizzing through space close to the speed of light and claim that they are stationary and it is Person B who is moving. Likewise, Person B can claim that they are stationary and it is indeed Person A who is moving close to the speed of light (along with the earth which Person A is standing on). What I don't understand is where kinetic comes into this. If Person A is stationary they have no kinetic energy and they perceive Person B to have a high amount of kinetic energy. But at the same time Person B perceives them to have a high kinetic energy similar to that which Person A thinks Person B has. They can't both have high kinetic energy AND no kinetic energy can they? I hope that makes sense and I've not overcomplicated it too much. Sorry if this is a stupid question or has an obvious answer, it's just something that has been bugging me! Thanks.