The sponge is an interface that allows a clear link to beestablished between gesture and sound in electroacousticmusic. The goals in developing the sponge were to reintroduce the pleasure of playing and to improve the interaction between the composer/performer and the audience. Ithas been argued that expenditure of effort or energy is required to obtain expressive interfaces. The sponge favors anenergy-sound relationship in two ways : 1) it senses acceleration, which is closely related to energy; and 2) it is madeout of a flexible material (foam) that requires effort to besqueezed or twisted. Some of the mapping strategies usedin a performance context with the sponge are discussed.