One of the sections in the Learn Responsive Web Development from Scratch course teaches us how to use Bootstrap from the basic code to build a web page.
As I followed through the video, there is one part whereby the trainer removed the CDN version of jQuery at the bottom of the basic template which you can find it from getbootstrap.com, and replaced with a downloaded copy of jquery.js from jquery.com. The placement of the script sequence in the <head> tag section is important as the compiler reads through the first script we put in the <head> tag.
The compiler will run the jquery.js file first and follow by the bootstrap.js file where some of the controls are being overridden and you can see the effects according to the sample shows in Bootstrap website.
In any cases where you accidentally put the jquery.js script below the bootstrap.js script, the Bootstrap is not working accordingly. So, do not panic!
Oh ya, just in case, you do not know what I meant by CDN version of jquery.js file, it is a file hosted in the distributed servers (networks) that delivers the web content or file to users based on geographic location of the user.