Shnoop offers the Hydrofarm Thirsty Light Snowflake Christmas Tree Digital Water Moisture Sensor 2-Pack, model no. HGTLSF, for $10. Coupon code "BFHYDRO" cuts it to $5. With free shipping, that's the lowest total price we could find by $11. (However, we saw three sensors for $7 last month.) It alerts you when your Christmas tree needs watering.