What's the opposite of O.K.? I'm sure you'll slam this one!

I wake when clouds go poof, a stripe parade in the sky, Splashing colors for the brave who dare to look up high. I’m faster than a cereal chase, brighter than a hero’s hair, A skybound candy ribbon twisting through the air. Catch me quick, or I’ll be gone, leaving wonder in my trail. What am I, giggling in sunlight, too wild to ever fail?

I'm so slim, a slice looks obese next to me.

Average everyday. Terrific mail. Terrible frisbee.

When at sea I’m on the left, when at dock it’s where I’m at. In a glass I’m sweet and dense, apocryphally I’m the start of posh. I’ve carried this riddle far enough, what am I?