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

False:

    我吃了两两米饭。

    Wǒ chī le liǎnɡ liǎnɡ mǐfàn.

True:

    我吃了二两米饭。

    Wǒ chī le èr liǎnɡ mǐfàn.

 

二用于表示度量衡单位的量词“两”前，因为读音的关系，不能用“两”，只能用“二”。

Before 两 which is also a measure word of weight, 二 rather than 两 is used for pronounciation reason.