Recently, I was asked whether an asset that is fully depreciated could continue to be depreciated because the asset was still in service. The answer is "no" because once an asset is fully depreciated there is nothing left to depreciate. The accounting concept of "book value" needs to be understood in order to appreciate why the answer is "no". An asset is purchased at a given price and recorded as such. At that moment in time, the purchase price is the "book value" of the asset.
When depreciation is applied to the asset, the book value of the asset is decreased. The process of depreciation is to remove a portion of the book value of the asset and expense it via depreciation. At a given time, you can figure the book value of an asset by subtracting the depreciation that had been accumulated up to that point in time.
If you sell a fully depreciated asset, or an asset with a zero book value, then the sales price is all gain. If you discard an asset that still had a book value, then there is a loss on the discard. If you have a gain or loss on a trade, the gain or loss is deferred by rolling the gain or loss into the book value of the new asset.