When you write a handler for a function, you specify statements that compute and return a value to the handler that calls the function. Each function handler has the following form, where the italicized words are placeholders:

function functionName
    statements
end functionName

HyperCard has many built-in functions, but you can also write your own:

on mouseUp
   put square(5) into the Message box
end mouseUp
function square x
   return (x * x)
end square

The function square receives a number through its parameter variable, x. It then returns the value of x * x to the handler that called it (mouseUp) using the return keyword.