As far as I have encountered different phrases involving a lot of ,a noun succeeding this phrase can be either singular or plural (but not both, obviously) such as
a lot of books Plural version
a lot of rice Singular-like version
What does this tell you? :) That uncountable nouns are the ones present in their singular or rather, their only available form whereas the countable nouns are written in their plural forms.
Who wants to count hair? So, it is often treated as uncountable, but technically you can count hair if you really had nothing better to do. That is why you will see it used as singular or plural in Dutch. I agree that certain expressions will use one or the other in Dutch. In English it is mostly used as a singular word, but in Dutch "haar" is also used for "her" and "its" as well as for "hair". I imagine you would rather say "haar haren" as opposed to "haar haar", or am I wrong? You do say "mijn haar". http://dictionary.reverso.net/dutch-english/haren
http://dictionary.reverso.net/dutch-english/haar (scroll past the possessives for hair)
I agree to disagree. Just because someone has time to count doesn't make English to decide upon those nouns accordingly, because if that was the case even rice could be countable. Similarly hair is considered to be uncountable unless there is the countable form of it present on the floor or on someone's hands.
Rice is different because you count "grains of rice" or "rice plants". Hair can mean each individual strand or the covering of all them on your head. The same word is both countable and uncountable depending on which definition you mean. We actually do agree that "hair" is mostly used as the uncountable form. Both forms exist though in English as well as in Dutch, but the Dutch use of the forms may be different from ours. My understanding is that Duolingo is accepting both singular and plural Dutch versions in many situations. http://www.merriam-webster.com/dictionary/hair