It's long been the the norm for schools and universities to insist we hand in papers with double-spaced text. Don't waste time wading through the Format options, because there's a way to single-space or double-space a document with one quick shortcut.
Most people (including myself) use the Format options on the Home tab of the ribbon (or Format > Paragraph in older versions) to change the spacing of a document, but there's a much simpler way. You can press Ctrl+2 to double-space an entire document (or Cmd+2 if you're on a Mac). Ctrl+1 single spaces it, and Ctrl+5 puts it at 1.5 spaces. Simple, quick and hassle-free.
Sadly, this doesn't work in Google Docs or LibreOffice, and there don't seem to be any equivalent shortcuts for those alternatives. But if you're using Microsoft Word, you've got it made in the shade. Hit the link to see a full list of Word shortcuts, courtesy of Microsoft, or check out a handy printable cheat sheet.