keeping green house warm

so there is an easy way to keep your green house from freezing,

water does a great job of having a high thermal mass,
so here is a 55 gallon drum full of water with a fan on it.

it was frost free inside when it looked like this outside (poor frozen flowers)

if you don't have a green house, the water still has high thermal mass, 

so watering things before a light frost can get them through it.