1 sentence

1. 这些按钮在手机正在使用时点亮，否则不可见。

   zhè​xiē àn​niǔ zài shǒu​jī zhèng​zài shǐ​yòng shí diǎn​liàng, fǒu​zé bù kě​jiàn.

   These buttons light up when the phone is in use and are otherwise invisible.

   Source: CCMatrix