I'm wondering what's the difference between the groupBy and the ToLookup Extension Method.

Let us have a List of objects like this:

```
public class Person
{
    public uint Id { get; set; }
    public string Name { get; set; }
    public DateTime Birthday { get; set; }
}

List<Person> People { get; set; }

```

Now i can use the Extension Methods above:

```
var groupedPeople = People.GroupBy((x) => x.Id);

var lookupPeople = People.ToLookup((x) => x.Id);

```

What's the difference between those statements?

Thanks in advance.

Marco B.