Q: How does a mathematician deal with constipation?
A: He works it out with a pencil.
When Newton invented his calculus (fluxions), he wrote them with dots over the letters of the variables (fluents) he was deriving. So, x with a dot over it meant the rate of change of x with respect to time. Pretty soon the equations started to fill up with dots, looking like flyspecks on the page.
That was before (pronounce this!) DDT.
Separate names with a comma.