Nouns that can be countable or uncountable
Some nouns can be either countable or uncountable. For example, a potato is a separate, individual thing, but potato cannot be counted.
COUNTABLE
I'm peeling the potatoes. Melanie baked a cake for David. Vicky was eating an apple. Someone threw a stone at the police. There's a hair on your shirt.
UNCOUNTABLE
Would you like some potato? Have some cake/a piece of cake. Is there apple in this salad? The house is built of stone. I must brush my hair, NOT hairs
Often the countable noun is specific, and the uncountable noun is more general.
COUNTABLE
Rugby is a sport. (= a particular sport) That's a nice painting over there. We heard a sudden noise outside. John Lennon had an interesting life.
UNCOUNTABLE
Do you like sport? (= sport in general) Paul is good at painting. Constant noise can make you ill. Life is complicated sometimes.