If you hold the incense above a glass tube that is over cold water, the smoke from the incense is likely to cool down as it comes into contact with the cold surface of the glass tube. When the smoke cools, it tends to condense into visible particles, forming a mist or fog-like appearance. Some of the smoke may even turn into tiny droplets of water. The presence of cold water can facilitate this cooling process, causing the smoke to transform visually.