Paper 1 October/November 2011 PHYSICS (0625/11) IGCSE

A shopkeeper pours rice into a dish that hangs from a spring balance. He records the reading.
A customer buys some pasta. The shopkeeper notices that the reading on the spring balance, with just pasta in the dish, is the same as it was with just rice in the dish.
Which quantity must be the same for the rice and for the pasta?