Figure 2 - Instructor (left) and student (right) adding water to rocket. The rocket was made by several amount of plastic bottles. To make some pressure, the rocket is filled (about quarter of the body) with water
<a href="//commons.wikimedia.org/wiki/User:Joo_Nath" title="User:Joo Nath">Joo Nath</a>
CC BY 4.0
<span class="int-own-work" lang="en">Own work</span>