Think of it like rubbing two sticks together to start a fire -- You need to put enough energy into a system to maintain the subsequent high-energy reaction. In

Think of it like rubbing two sticks together to start a fire -- You need to put enough energy into a system to maintain the subsequent high-energy reaction. In a fusion reaction, we're supplying the fuel (Deuterium/Tritium/Helium instead of wood) and the spark (electromagnetic/laser instead of fire) to provide enough energy for the fuel to sustain an ongoing energetic reaction. The primary difference is instead of the fuel disassembling due to the reaction (carbon chains in the wood combining with oxygen to form heat, CO2, ash and H2O), in a fusion reaction the energy is used to combine two molecules (usually two isotopes of hydrogen combining to make a single helium atom with excess energy/heat).