 Many other small organisms have been used experimentally as living models of mathematical equations or simulations (e.g., mites, beetles). But microbes are particularly tractable in that they are easy to maintain and propagate in a laboratory, especially compared to other classic model organisms like mice. This is partly because they are so small: huge populations of them can be kept in tiny spaces. It is also because some species do not need much looking after compared to other laboratory organisms; pop them into a flask of growth medium (a simple mix of a few chemicals) or smear them on an agar plate, and their populations will grow exponentially. Many laboratory cultured microbes have very short generation times compared to larger organisms. Some bacteria can produce tens of generations a day if not resource constrained, which allows for long-term evolutionary studies over reasonable human research timescales.