When an expression that consists simply of a variable name is evaluated, the value of the expression is the value of the named variable. The evaluation of a variable reference expression has no side effects.
So, after
(define key "Paul Evans")
the value of the expression key is the string value "Paul
Evans". If key is then modified by
(set! key 3.74)
the value of the expression key is the numeric value 3.74.
If there is no variable with the specified name, evaluation of the variable reference expression signals an error.