Why "al sol"

Question

In the test the question is "Coloca las plantas _____ sol." I understand that "al" is the correct answer as far as a contraction, but why do you use "a" in this instance? Why is it not "en el sol"?

Answer

Hola Andrea,Here's what I think:If you were to use '...en el sol", in Spanish it would translate as "placing the flowers in (inside) the sun". By saying, "Coloca las plantas al sol" it's more or less saying, "Place the plants to the sun", in other words, "to face the sun". This is just my opinion though Andrea.This reminds me of the time I was on holiday in Italy...I saw a field full of sunflowers and they were all facing exactly the same way, towards the sun! :)

Answer

Hola Andrea
Clara's explanation makes sense. If we say "en el sol" it'd be literally "in/on the sun", but to be honest, we do sometimes use it that way with the obvious meaning "receiving the light/heat from the sun". For example, if you've been to the beach you could then say: "Estuve en el sol dos horas" or "Estuve al sol dos horas" - although the second one is a lot more common, you can also hear the first one meaning the same. 
The same goes for "a la sombra" and "en la sombra" - using "a la sombra" is more common but you can easily hear something like: "Ponte en la sombra, que hace mucho calor" (Place yourself/Stand in the shade, it's very hot.)
After trying to think about the specific nuance on these I came to the conclusion that these are simply set expressions using "a" more frequently, so I wouldn't worry too much about getting it wrong by using the wrong preposition. 
Saludos
Inma