Designed to let you know when the water on your Christmas tree is getting low, there are probably lots of other applications for this device related to fish tanks or hydroponics. It can be strapped to the trunk of your tree with cable ties; loops on the unit are provided for this purpose. When the two-pronged probe is in water, nothing happens. When the water evaporates, and the probe is dry, a piezo beeper sounds and a red LED flashes. Operates on two AAA cells (not included). ME530