Prepositions: Choosing the right one

Question

For example: Nosotros nos reímos siempre con esa película. We always laugh at that movie. I would have never known to use "con." I would have chosen "a."

And, Ustedes se ríen de los niños. You laugh at the children. Again, I would have chosen "a" instead of "de." Why "con" in one sentence and "de" in another?

Will there be lessons on how to choose the correct preposition, or is it a matter of memorising the conjunction with the noun?

Accepted Answer

Hola Dianna
The verb "reírse" can be used in slightly different ways. So, if you say you're "laughing at someone" you always use the preposition "de" in Spanish:

Los niños se reían de ella porque era muy torpe.
The kids laughed at her because she was very clumsy.

The other sentence is using "con" with a different meaning. If you say:
Nosotros nos reímos siempre con esa película. 
this doesn't mean that we laugh "at" the movie, this means that we always laugh every time we watch / while we watch that film because it has lots of funny scenes. Here there is no "mockery" meaning as before. 
Prepositions can change their meaning depending on which verb is using it. This is what also happens with phrasal verbs in English, a specific preposition goes attached to a specific verb with a specific meaning. 
I hope this clarified it.
Saludos