Slime and putty should soften after you knead it. If the slime is too hard and crumbly, add it to a bowl with a tablespoon of peanut butter and heat in microwave for 15-30 seconds. Stir and knead. Add cornstarch to prevent stickiness until you achieve desired texture.
Store in the refrigerator in an airtight container. Edible up to 2 days.