This add-on for the Build N" Balance Set (sold separately) provides a rocking function. The children have to transfer their weight as they move from top to top. The Rocking Plank moves approximately 2.5" up and down as it rocks. Dimensions are: 28.5" Long x 5.1" Wide x 3.9" High. (Note that with this offering, you only get the "Rocking Plank", you do not get the Tops you attach the Plank to on each end. Even though the photo shows the Rocking Plank connected to the Tops, you only get the Rocking Plank.)