How can I figure out how to merge 2 pdf files?

I need to combine two separate PDF files into one document for class. I tried dragging one onto the other on my desktop, but nothing happened. How do I actually get them joined together?

Honestly, just dragging them onto each other won’t work by default. If you are on a Mac, you can do it right inside Preview, otherwise just drop them into a free site like ilovepdf to combine them real quick.