How to keep text inside the parent?

Question

Lets say take this as an example:

Answer 1

Warning: Probably, your desired behavior is JoshC's answer, but my answer keeps text inside the parent too (in another way).

Use overflow different than visible (i.e auto, scroll or hidden).

On most cases, I recommend auto:

#container {
    margin:10px;
    width:400px;
    border: 2px solid red;
    overflow: auto;
}

Demo