Words like down, in are not always prepositions. Compare:
I ran down the road. He's in his office.
Please sit down You can go in
In the expressions down the road and in his office, down and in are prepositions: they have objects (the road, his office).
In Please sit down and You can go in, down and in have no objects. They are not prepositions, but adverbs of place, which modify the verbs sit and go.
Small adverbs like this are usually called 'adverb particles' or 'adverbial particles'. They include in, out, up, down, on, off, through, past, away, back, across, over, under. Adverb particles often join together with verbs to make two-word verbs, sometimes with completely new meanings. Examples: break down = 'stop working'; put off = 'delay', 'postpone'; work out = 'calculate'; give up = 'stop trying'. For information about these verbs, see the next section.
Best Valentines Day Gifts
Make Your Own Beer Mustard Kit
For men who love beer and mustard, this kit is a creative Valentines Day gift idea. Each bottle of ingredients has a anddate of birth so you know your spice is still fresh. Just add 1/4 cup of white or apple cider vinegar and 1/4 cup of his favorite beer (or cider), and hell have a mustard thats bitingly intense. If hes more of a tea guy, perhaps this make your own chai kit would be a more interesting option.