Moooo

I was recently in a full car going someplace of no significance to this story when somehow the conversation turned to cow-tipping and the merits of going under the cover of darkness rather than under the mid-day sun. I was amused by the fact that my passing thoughts on the subject that had existed for many, many years were in fact not the same as the other occupants in the vehicle. I had always assumed you would go cow-tipping at night so that you wouldn't stick out like a sore thumb in some stranger's field and have an angry shotgun-wielding farmer running after you. After some chuckling went around, I was informed that "No, silly" you go cow-tipping at night because that's when the cow is sleeping. I never thought it really mattered if the cow was awake or not, and it turns out it doesn't. It's all urban legend anyway. Which makes the most sense of all.

Cows are really heavy, whether it's light or dark out.

http://www.truthorfiction.com/rumors/c/cowtipping.htm