GUU is a button mushroom with little slit pockets to hide rewards. The round, unstable bottom adds on to the challenge and they are just perfect to play fetch with too. Longer pieces of treats work best for the GUU. We do not recommended using kibbles or pieces that are too small. If the treat is completely inside the pockets, your pet might get frustrated (since they have paws, not fingers)! Pockets are designed to be easily pulled out for washing. Kitty owners can insert catnip in the pockets too. Small: 5" or 13cm, suggested for breeds such as chihuahua, italian greyhound or cats Medium: 8" or 20cm, suggested for breeds such as shiba, corgi or poodles