Word-level bigrams/trigrams in tsvector

I'm wondering how I can store word-level bigrams/trigrams in a tsvector
that I can query against. I was expecting the final query to match "the
air" and return the one tuple to me.

For instance:

postgres=# create table docs (a tsvector);
CREATE TABLE
postgres=# insert into docs (a) values (strip('''the air'' smells ''sea
water'''::tsvector));
INSERT 0 1
postgres=# select * from docs;
a
--------------------------------
'sea water' 'smells' 'the air'
(1 row)

postgres=# select * from docs where a @@ to_tsquery('''the air''');
a
---
(0 rows)

Thanks, Alan