The Dilemma: For reasons that are really none of our business, you’re extremely curious about nuclear reactions. Hey, that’s cool. We’re here to help.

People You Can Impress: nuclear physicists, environmental protesters, and just by correctly pronouncing NEW-clee-er, some American presidents

The Quick Trick: Fusion fuses elements lighter than iron. Fission divides elements heavier than iron.

The Explanation:
These two forms of reaction are called nuclear because the big stuff (that’s a technical term) all happens inside the nucleus of an atom.

Let’s put it in the simplest terms: Fusion works by smashing atomic nuclei together to create heavier nuclei. In order to make this happen, you have to heat things up a bit—say, a few million degrees Celsius. Fusion is promising as an energy source (and potentially dangerous) because it’s exothermic—it produces more energy than it requires to start it and is therefore self-sustaining. How much energy can fusion produce? Well, our sun’s been working for several billion years just fine on fusion.

Fusion is difficult to achieve because of something called the coulomb barrier. This is the energy required to overcome the electrostatic force that repells two nuclei from each other. What makes fusion tricky is that the best fuels, the ones with the lowest coulomb barriers, are the least stable. They are isotopes of hydrogen—deuterium (2H) and tritium (3H)—so named because they have two and three neutrons in their nuclei, respectively. We are not yet able to create a fusion reaction and contain it to use as energy. In fact, the ones we’ve created that aren’t contained are called hydrogen bombs. This has physicists looking for an alternative that requires much less energy to start and contains the Holy Grail of nuclear science known as “cold fusion.”

Fission, on the other hand, is the complete opposite: Very large nuclei are split to make smaller ones, releasing energy (and a boatload of radiation) in the process. Again, the best fuels are the most unstable: Isotopes like uranium-235 or plutonium-239 don’t occur naturally. Fission is used both for nuclear power plants and nuclear bombs.

To get fission going, you bombard the nucleus with a free particle, like a neutron or photon. The nucleus splits, releasing energy and more neutrons. If the split produces enough neutrons to keep the reaction going, it increases exponentially, and you reach critical mass.

Cold, Hard Facts
In 1989, two researchers at the University of Utah held a press conference and made a stunning announcement: They’d achieved nuclear fusion at room temperature, which offered the promise of an endless supply of cheap energy to the entire world. One problem: The two researchers had actually, as it turns out, not achieved fusion at room temperature. They hadn’t achieved it at all. In the years since, neither the original duo (who still claim the announcement wasn’t a hoax) nor other researchers have managed to replicate the initial results.