This fruit has rotted.

We went down the river by boat.

He remained silent and didn't say anything.

He puffed out a breath after climbing the long stairs.

After running up the stairs, he breathed heavily.

She loves writing fan fiction.

Recently, his popularity has been increasing.

I could see the beautiful sea from the porthole.

I went to the stationery store to buy new letter paper.

A mass noun refers to a noun that cannot be counted.