Solid, in physics and chemistry, one of the three states of matter. The other two states of matter are gas and liquid. A solid tends to maintain its shape without a container. Some solids are harder than others. True solids are crystalline; that is, the atoms of which they are composed are arranged in definite, three-dimensional patterns. They have definite melting temperatures. Some substances, such as glass and tar, that appear to be solid are amorphous—they do not have a crystalline atomic structure. These substances soften and melt over a range of temperatures; they are not true solids.
Over the years, nuclear reactors have been viewed as both a miracle and a menace. How does a nuclear reactor do its job? And what happens when something goes wrong?
Twenty-seven kilometers is more than five 5K races. Most humans aren't interested in running that much, so why do a bunch of speeding protons require that considerable distance?