When communicating in Chinese you may often make mistakes using Chinese grammar like:

False:

    有一只鸟掉进缸。

    Yǒu yì zhī niǎo diào jìn gāng.

True:

    有一只鸟掉进缸里。

    Yǒu yì zhī niǎo diào jìn gāng lǐ.

“缸”是个表示实体的名词，不能单独用来作“掉进”的宾语。须在“缸”后加上方位词“里”，表示处所。

As a noun denoting entity, 缸 can not be the object of 掉进 just by itself， but ought to be followed by the noun of locality 里， to show location.