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