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Somnom Stones are special stones that appear in Somnom Woods in Mario & Luigi: Dream Team. When the Bros. free a Master Pi'illo, it gives them a Somnom Stone, which the Bros. can use to create a track within Pi'illo Temple to reach related areas and find more Master Pi'illos.
When they near the ideal spot for the Ultibed to be place down, they see a large gap that must be crossed, and learn that to get to the other side they have to find the five remaining Master Pi'illos which hold the Somnom Stones. Once all of the Somnom Stones are collected, the Bros. can assemble a long enough track to reach the other side and hit a button with the hammer to assemble a complete set of tracks, making the Somnom Stones obsolete.