Last month, big software company Adobe had their computer systems hacked - and many millions of users' passwords were revealed.
Researchers looking at the passwords found that many of them were really easy to guess - making it easy for criminals to get into people's accounts.
The Adobe leak showed almost 2 million people had the simple password "123456" for their accounts.
The second most popular was "123456789" and the third most popular was simply "password".
Common password mistakes
James Lyne, from IT security company Sophos, says this is the first of two common mistakes people use when choosing passwords.
If your password's really easy to guess, it makes a hacker's job much easier, meaning they can access your account and could lock you out - or even worse, steal stuff.
Another common mistake, says James, is when people use the same password across all their different online accounts - so if a hacker guesses it, they can get into everything.
Top tips: Choosing the best password
- Don't choose a single word, like "password" - choose a combination or phrase instead.
- Adding numbers or symbols can really make a difference too: "ilovenewsround123" is a much stronger password than "newsround".
- A good method is to think of your favourite song lyric - which makes your password much longer and really hard to guess.
- It's also important to use a different password on each website, instead of recycling the same one again and again.