[Gurthang]

Most people think that some guy discovered electricity by flying a kite with a key on it. Gandalf had that whole 'electric glow' figured out way before then.

OR

Gandalf: "I have an idea!"

OR

Theoden: "Does he always glow like that?"
Aragron: "Only when he's starved for attention... so yes, he does always glows like that."

OR

Theoden: "Uhm, Gandalf, you forgot to paint the bottom of your hat."

OR

Gandalf performs his electric eel impersonation.