A caterpillar goes to sleep inside a special blanket called a cocoon, and while it rests, it changes into a butterfly. After some days, it wakes up and spreads its colorful wings to fly.
Or you can say: The caterpillar wraps itself in a cozy shell, where it grows and transforms. When it is ready, it breaks free and becomes a beautiful butterfly.
Another way: The caterpillar makes a tiny sleeping bag, and inside, a magical change happens. Then it comes out with pretty wings and can fly in the sky.