Consider, for example, the fact that Hiroshima was leveled by a 13 kiloton weapon, resulting in an estimated 80,000 deaths. By comparison, the modern W80 nuclear warhead-- one of the most common in U.S. active deployment-- has a maximum yield of about 150 kilotons. This weapon is so powerful that it can completely wipe out a typical medium-sized city, but at the flick of a switch, the warhead's potency can be reduced to as little as five kilotons. This handy feature is called Dial-a-Yield, and it allows nuclear stockpiles to take advantage of the one-size-fits-all approach.
In some cases, weapon yields are adjusted prior to use by replacing portions of the fissile material with lead tampers, thereby reducing the potency. One such weapon was Tsar Bomba, the most powerful bomb ever exploded, which was dialed down to 50 megatons from its maximum of 100 megatons by replacing a fission stage with lead. But modern nuclear missiles can be remotely adjusted to the desired destructive power, even while the weapon is in flight.
The key to this feature is a method known as "boosting." Early on in atomic weapons development, nuclear weapon scientists realized that injecting a bit of tritium gas into the center of a nuclear device could increase its destructive power tremendously. Tritium is a radioactive isotope of hydrogen, and although cosmic rays create very small quantities of tritium in the atmosphere, the only way to create usable quantities of the gas is inside nuclear reactors, by using neutron activation of lithium-6.
Nuclear boosting was first tested in 1951 in a U.S. fission bomb test codenamed "Item." The test weapon was originally a 20 kiloton device, modified to include tritium in its core. When detonated, researchers found that the boosting method increased the bomb's yield from 20 kilotons to 45.5 kilotons, more than doubling the destructive power. The amount of tritium used was only a few milliliters.
This principle was later improved upon and integrated into nuclear weapons by leaving a vacuum-sealed hollow in the center of the fissile material, and including a valve which could be opened to release tritium into the core. This allowed one weapon to offer a wide range of power, offering a small-scale "tactical" solution in the same package as a large-scale city-flattening missile... all at the twist of a knob. Eventually an electronic control was added to allow this valve to be opened remotely.
The drawback to using tritium gas in this way is that production and storage of tritium are serious problems. Tritium can only be produced inside nuclear fission reactors, so its supply is extremely limited. Moreover, its half-life is only 12.5 years, which causes tritium-boosted weapons to rapidly degrade in destructive potential while in storage. In fact some nuclear warheads cannot reach a supercritical reaction without a certain amount of radioactive tritium, which means that these weapons don't have much of a shelf-life.
Most modern nuclear weapon engineers now consider Dial-a-Yield boosting to be a design flaw due to the extreme undersupply of tritium gas, and tritium's fleeting radioactivity. But history has shown us that humans are far too clever to be outdone by nature, and soon we'll have even more efficient ways of killing lots and lots of people.