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Yes middle school, she's a daddy's girl can you tell hehehehe. 
Carah,  I use 2 different heaters, I have 3 or 4 of a commercial type water heater bases that are a large galvanized pan with a sealed flat heat element in the bottom of it, all you do is turn it upside down sit the your waterer on it and plug it in. It is thermo regulated and automaticly comes on & only keeps the water at around 40 degrees or so. Works great but some may think @ $28.00 it may be too pricey, I like them and they are super safe and easy to use and long lasting durable.
The other option I came up with this year and will be making about 6 this month, is a similar design but I am just going to take my waterer bases whether it be plastic or metal I have both. And attach short 1'-2' lengths of what is refered to as heat tape, they are those yellow cords you wrap around pipes to keep from freezing. See where I am going with this!!!!, they are super cheap at around $5-7.00 for the short ones they last forever, just duck tape them to the pan bottom. They have a light on one end to let you know it's on, these are safe, use very little tricity easy to use just plug and go.
                                                                 AL