Fluid effect in html/css/js

Question

How can I creat the following effect : when a button is clicked a new page will be showed in a way that drop down like a fluid not as a solid box

Answer 1

https://ru.stackoverflow.com/questions/822070/%D0%9A%D0%B0%D0%BA-%D1%81%D0%BE%D0%B7%D0%B4%D0%B0%D1%82%D1%8C-%D1%8D%D1%84%D1%84%D0%B5%D0%BA%D1%82-%D0%92%D0%BE%D0%B4%D1%8B/825600