In the earlier days when you could trust almost everyone, you just locked your bicycle with a key to make sure it would not be taken. Today you have to be careful. We have many people that rely on their bike perhaps as a courier to bring the ordered pizza, or as a quicker way to get through the traffic.
I noticed that today you must chain your bike to something fixed and firm, like this young main did. And I hope he does not lose the key to the chain, otherwise he will have a problem.