What adhesive should you use to tile over tile on a back splash?

When tiling over tile on a backsplash, it's best to use a high-quality thin-set mortar specifically designed for this purpose. A modified thin-set with good adhesion properties is recommended, as it will bond well to the existing tile surface. Additionally, ensure the old tiles are clean, dry, and free of grease or dust before application to achieve optimal adhesion. Always check the manufacturer's instructions for compatibility and application guidelines.