Weak entity and one-one or many-many relationship
If you are running a video rental store, then whether or not your children eat depends very much on a video appearing in as many reservations as possible.  

In your example, RESERVATION is a weak entity which relies on both the FK to CUSTOMER and the FK to VIDEO in addition to a further attribute, perhaps date, to comprise its primary key.

It is not possible for a M:N relationship to be supporting for a weak entity (although the intersection table which resolves a M:N relationship can be a weak entity).  This is because a supporting relationship needs to be to a particular entity object.  A set of entity objects can't identify something in relational algebra.

It is possible for a 1:1 relationship to be supporting for a weak entity, although this would be rare "in the wild".  I could imagine it coming up in the context of situations that would typically be 1:M, but where there is a business rule that restricts the child to one at a time.