The primary objective of this experiment is to investigate if the SWI illusion is present in altered gravity environments. Since weight cannot be estimated in 0G, investigators have developed a procedure during which the subject is asked to estimate the objects' mass by shaking them rather than lifting them. Our results in 1G indicate that a size-mass illusion (SMI) is also present in 1G. Our hypothesis is that the previously observed alteration in 3D visual perception in 0G, particularly in the height of 3D objects, as well as the change in proprioceptive inputs, will be responsible for changes in the SMI in 0G and 1.8G compared to 1G. In a separate proposal, investigators will also propose to perform this experiment using the ESA short-arm human centrifuge (SAHC), with subjects exposed to various centrifugal force levels, to further investigate the role of proprioceptive cues in the size-mass illusion.