After I have presented this concept and obtained their intellectual consent, I point out that there are two prepositions in Spanish that distinguish these two meanings -- plus others, but that this example is a good place to start. I tell them that por is the preposition that would be used in Spanish in the first case and para in the second.
Next, I take a risk -- hoping that I have a class with powers of abstraction. I draw two circles on the board. Through one, I draw an arrow, all the way, through its diameter. I also draw an arc around part of its circumference and one within. I also draw a curving path around inside the circle. I draw an arrow pointing at the other circle.
I label the first circle por and the second para. As I do so, I tell them that por is static, and para is dynamic. I then tell them that para shows destination or deadline -- thus it refers to time or space. I write El avión va para Chicago and La tarea es para el lunes.
After these examples, I begin writing the usual chart showing the uses of por: through, along, by, in (spacial) and during (temporal) -- as I trace the lines I drew to symbolize these meanings on the circle labeled por. Then I add a few phrases beneath that circle to show means (por teléfono, por tren), exchange (cuatro monedas de a 25 centavos por un dólar), and a few stock phrases, such as por el bien de, por el amor de Dios, por favor, ¿por qué?, etc.
Under the circle representing the dynamic qualities of para, I write recipient (mom), direction toward (Chicago), in order to (I show para + infinitive in a sentence), unexpected comparison (¡Qué grande para un niño de siete años!), and intended purpose (Es una copa para vino).
Following this presentation, I do some oral work in English, asking which preposition would be used in Spanish.