The hash sign (which looks like # and is also known as a number or pound sign in the US) used to be a bit niche until Twitter came along. These days it's #YOLO this and #EdBallsDay that, and you can barely avoid the symbol that is widely known as the hashtag after its use on various social networks.
But how do you type a hash or hashtag symbol on a Mac? The PC keyboard has a dedicated hash key, but it's harder to find on a Mac keyboard. On a standard UK-layout Mac keyboard, indeed, the hash sign doesn't appear anywhere, so we'll have to get crafty with the Alt (Option) key to make our Twitter hashtag.
To do a hashtag on a UK Mac, press Alt + 3. It's as simple as that.
If you're using a US Mac keyboard, meanwhile, things are easier: it's the secondary option for the 3 key (you'll see a # above the 3) so Shift + 3 will do the trick.
For more sneaky Mac keyboard tips, see Mac keyboard shortcuts you need to know.