Create a cross site lookup field without code

Out of the box you cannot create a cross site lookup field in SharePoint. You must create a new field type (code) to do this.
However the other day I came across a easy workaround where you don't have to code anything:

1. Create your lookup list on the parent site
2. On the parent site create a lookup site column that looks into the list created in step one.
3. On the subsite create a list that should use the information stored in the list on the parent site.
4. Add an existing column to the list (the column created in step two).

See also: Creating a cross site lookup field without code