I tested this myself before giving it to my one year old. It’s a soft and flexible silicone material, very durable and thick. So it is of good quality. If it’s turned upside down, obviously it’ll leak (whether the sippy cup nozzle or straw cup nozzle), and if it’s upright, it won’t leak as long as you don’t squeeze the sides of the cup. I also held the cup sideways and no leaks! My littles only use straw bottles or open cup itself, so I’m assuming since the bottles we use are a hard plastic. My one year old was squeezing the cup, and her whole front side was soaked with water. It’s a good design and bbluv is a trusted brand that we use in our house. But because my littles all have used open cup and straw bottles since 6 months old, it isn’t meeting our expectations for this specific product. If your little won’t squeeze the sides of their bottle or cup, then this is the perfect one for you! Arrived quickly, and nicely packaged!