Best short riddles

Because you can see right through them.

Your number is 5.

Cell, Sell.

The owner got tired of the (w)hole business!

Manhole covers are round so that they won't fall through the hole into the sewer below them. No matter how you turn the cover, you won't be able to push the cover through the hole. However, if you were to have square manhole covers, you would be able to rotate the cover such that one of the edges of the square cover is lined up with the diagonal line of the square hole, which would allow the cover to fall through, causing countless problems that the general public would rather avoid.

Where is the popcorn?

The gold fish.

The letter 'n'. Throw out the letter n from the word window and you have widow. Bring back the word n to door and you have donor.

Because it was not peeling well.

Baking soda.