When Palpatine and Yoda were kids, Palpatine threw some lightning at Yoda, and Yoda melted. Yoda is made out of snow. Green snow.
This is proven in Episode III because, as all village idiots know, lightning melted Sidious face, due to the fact Sidious is made from wax and lightning clearly has the power to perform plastic surgery.
I swear, people are f*cking mental.