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.
Uncommon Fruits and Veggies
The durian may be the world most mysterious fruit.First there the outer husk, which is covered with thorns and must be broken through to get to the yellow fruit inside.Then there the smell.The durian is known for its absolutely horrendous odor, which eaters must overcome to get to the flavorful fruit.While many think the inner meat is delicious, people often disagree about its flavor and liken it to something that they have never experienced before.This is one of those foods for which the phrase either you love it or you hate it aptly applies.