This is a consequence of Einstein's theory of special relativity. Here's why:

* Energy and Mass are Interchangeable: According to Einstein's famous equation E=mc², energy (E) and mass (m) are equivalent. This means that energy can be converted into mass and vice versa.

* Kinetic Energy and Mass Increase: As an object accelerates, its kinetic energy increases. This increase in kinetic energy manifests as an increase in the object's mass.

* Relativistic Mass: The mass we normally experience is called rest mass. However, as an object approaches the speed of light, its mass increases significantly due to its increased kinetic energy. This is referred to as relativistic mass.

* Approaching, Not Reaching: It's important to note that an object with mass can never actually reach the speed of light. As it approaches the speed of light, its mass increases exponentially, requiring an infinite amount of energy to accelerate it further.

So, in summary, the closer an object gets to the speed of light, the more massive it becomes.