Entity-Relationship Diagrams: Identifying which way a relationship flows
The flow of ERD relationships go in both directions. Cardinality expresses the rules governing the relationships between entities.  The 'one and only one' of classroom provides you with a verbal logic of - 'A classroom must contain..'.  The 'zero to many' furniture cardinality provides 'zero to many pieces of furniture.'  

Don't think of the furniture physically being in the classroom, just that a connection exists between the two.