Any fan to the show knows that Edd constantly wheres a sock-like cap on his head. He rarely takes it off, and if he does he'll be wearing something else on his head. In one episode, his cap was pulled off by accident. The audience couldn't see, but Ed and Eddy could, to which they responded "Cool" and "jeez Louise". Edd insisted that they tell NO ONE what was under there.
So one wonders, what IS under Double D's cap? What's the big deal?
Since seeing an unhealthy amount of these cartoons, and knowing some bit of how the show works, I've compiled my own list. In no particular order:

1. Sticky Notes.
2. That book about robot repair that you see now and then (once stolen by Sarah).
3. Gills (Eddy suggested this once to fool Ed into thinking Edd was a lizard crossbreed).
4. A notebook or journal.
5. Antennae (of the radio variety).
6. Sunblock.
7. Wet Naps ("messy, messy messy").
8. Rubber gloves.
9. An oxyacetylene torch.
10. A jawbreaker.

Allergic: He could be using the same matter compression technology he used to make a baking soda smoke screen. And the tounge thing definatly warrents further study, as all the kids have differently colored toungs. Maybe there's something in the water.