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## Expressions

Scientific simulations often keep track of several dozen variables as they run. However, only a small subset of those variables are usually written to a simulation database to save disk space. Sometimes variables can be derived from other variables using a variable expression. VisIt provides variable expressions to allow scientists to create derived variables using variables that are stored in the database. Expressions are extremely powerful because they allow you to analyze new data without necessarily having to rerun a simulation. Variables created using expressions behave just like variables stored in a database; they appear in the plot menu and can be visualized using VisIt's plots.