What's the difference between butter slime, fluffy slime, and regular slime kits?

Butter slime kits add clay to create a soft, spreadable, dough-like texture; fluffy slime kits include a foaming or whipping agent for a light, airy stretch; and standard kits make classic stretchy or clear slime. Several kits in this roundup specialize in one texture, while mega-kits aim to let kids make several types from one box.