We can say, "Look at the word" but not "Look the word at".
However, we can say "Look up the word " and "Look the word up" or "Look it up".
Why is this?
I've read somewhere that "at" is a preposition but "up" is not but adverb, but this doesn't solve my problem
I almost always decide wether or not I can put the word between the verb and the preposition just by a hunch.
Could anyone tell me how to explain this point of grammar? Thank you.