In elementary school, that is what you are taught, but in middle school+, you din't knock before you come in the class. Since you use the word colleague, I'll assume you're in college, and you wouldn't need to knock.
Tldr; False, but it's good manners
Putty can be different variations of bouncy, gooey or stretchy, based on the ratios of polymer and sodium tetraborate solutions.
This statement expresses certainty
