When you put a Christmas tree in water, you typically use a special solution called Christmas tree water. This solution contains a blend of nutrients and preservatives that help keep the tree fresh and green for as long as possible. The solution helps the tree absorb water and nutrients while also preventing bacteria and fungi from growing inside the tree. The solution can also help prevent the tree from drying out and becoming brittle, making it easier to keep the tree looking good and vibrant throughout the holiday season.