Negative temperature

In physics, absolute zero (0 K) is the coldest temperature. At that point, subatomic particles stop moving (entropy is at its minimum). Certain things can reach temperatures below absolute zero, known as negative temperatures. This is very difficult to do, and only very small objects can reach negative temperatures.
It might seem absurd, but things at negative temperatures are actually "hotter" than things at positive temperatures (above absolute zero). If something with a negative temperature comes in contact with a positive-temperature object, heat will go from the negative object to the positive. This is because temperature is a trade-off between energy and entropy. If you add energy to a positive-temperature object, it will "increase" in entropy. If you add energy to a negative-temperature object, it will "decrease" in entropy.
Many objects cannot achieve negative temperatures, because adding energy to them will increase their entropy. Only very small things discussed in quantum mechanics can reach this state.