In the equation f (x) = x sin x, each of the two sides—f (x) and x sin x—is an expression while f is a function. The function f is a rule that assigns to each number the product of that number and the sine of that number. Thus the function f defined by the equation f (x) = x sin x is the same function as the function g defined by the equation g(t) = t sin t. The expression x sin x (or f (x)) is different from the expression t sin t (or f (t)), since x sin x is tied to the variable x, and t sin t is tied to the variable t.