They're stuck to a surface that's provided by the environment that's around them, they're not swimming free in solution.

他们黏联在,周围环境的表面上,而不会在溶液中自由运动

耶鲁公开课 - 生物医学工程探索课程节选

For children, they have tennis. They have swimming, and it's all free for people.

有网球给孩子们玩。人们也可以免费游泳。

最美丽的公园 - SpeakingMax英语口语达人