Why my queryes doesnt not use indexes?

Hi guys, i am in trouble with some simple data that seem like doesnt
use any index.. and i dont know why.
My test database structure is this one:

------
CREATE TABLE users(
id BIGSERIAL NOT NULL PRIMARY KEY,
nickname varchar(50),
email varchar(50) NOT NULL
);

CREATE INDEX users_nick_index ON users (nickname);
CREATE UNIQUE INDEX users_email_uindex ON users (email);

INSERT INTO users (nickname, email) VALUES ('foo', 'foo(at)example(dot)com');
INSERT INTO users (nickname, email) VALUES ('bar', 'bar(at)example(dot)com');
-----

Now, i populated the database with around 5000 rows.

If i run that query:

------------
EXPLAIN SELECT email FROM users WHERE nickname = 'Errol'
------------

The result is:
------------
QUERY PLAN
Bitmap Heap Scan on users (cost=4.37..36.04 rows=15 width=28)
Recheck Cond: ((nickname)::text = 'Errol'::text)
-> Bitmap Index Scan on users_nick_index (cost=0.00..4.36 rows=15
width=0)
Index Cond: ((nickname)::text = 'Errol'::text)
-----------

So seem that it use the index.. but if i use the LIKE:

---------
EXPLAIN SELECT email FROM users WHERE nickname LIKE 'E'
----------
Postgresql dont use any index, and run with a seq scan:

---------
QUERY PLAN
Seq Scan on users (cost=0.00..112.05 rows=15 width=28)
Filter: ((nickname)::text ~~ 'E'::text)
----------

anyone can explain me why?

Im just structuring a db for a new application, if there is any
problem i'll like to solve it now ;)