Read this: Multi-layer printed circuit boards have trace layers inside the board. This is achieved by laminating a stack of materials in a press by applying pressure and heat for a period of time. This results in an inseparable one piece product. For example, a four-layer PCB can be fabricated by starting from a two-sided copper-clad laminate, etch the circuitry on both sides, then laminate to the top and bottom pre-preg and copper foil. It is then drilled, plated, and etched again to get traces on top and bottom layers.
Now answer this question, if there is an answer (If it cannot be answered, return "unanswerable"): Pressure is one thing you need to apply to make a multi-layer PCB; what's the other thing?
heat