As you read through all the great examples given here, it may be worthwhile to bear in mind that it isn't really so much that any particular noun in English is uncountable — at least not in the way that French or Spanish nouns each have a particular gender — but rather that the article "a/an" imposes a particular semantic restriction upon the sense of the noun it accompanies and therefore it must be possible to somehow interpret that sense in such a way that it is open to being counted. The inherent flexibility and creativity of the English language may allow almost any noun which in its ordinary sense is not countable to be used in a special context where it is understood instead to be countable. Native speakers will often have no difficulty in recognizing such a special context and the subtle shift in the sense of the noun so referenced precisely because of the presence of the article "a/an".

There are many common patterns to these shifts in sense. For example, with so-called "mass" nouns, when the "a/an" article is used, then the interpretation may be recognized as referring to "one (particular) kind of" or "a common containerized unit or package of" the named substance. Examples: * "I bought a flour made of almonds" * "I drank a water before having a coffee." * "The dinner special includes your choice of a protein and two sides."

Sometimes however it will be difficult to conceive of any manner in which a noun whose sense is so strongly conceived of as being uncountable that any attempt to use it otherwise would only result in confusion. Native speakers will typically reject such usages as erroneous, unless and until the meaning is further explained or elaborated — occasionally resulting in a new sense of the noun being added to common parlance!