In the first series, Sakura helps Kero-chan to collect the Clow Cards. To help them out, Sakura relies on the power of the Clow's Magic. Key of Seal: They Key of Seal is the only key that can open and seal the book of the Clow and make "the Agreement". Breaking the Seal: This means that Sakura takes the power of one of the Clow Cards. She does this by making an agreement with the Clow, and then the power of the Clow enters the key. After the help of Kaho Mizuki, Sakura becomes the "Mistress of the Clow". This means that she dosen't have to rely on the Clow's power any more, but on her own power. She creates a magic circle, which creates new cards, called the Sakura Cards (of course). With her being the Mistress of the Clow, her cards become even more powerful than the Clows themselves.