Quickly select all emails in Gmail

Some friends really like using Gmail and forward all their emails to their Gmail accounts. Over time, they have accumulated thousands of emails. Recently, they wanted to mark all these emails as read, but this posed a headache. The "Select: All" option at the top of the email list in Gmail only selects all the emails on the current page, not all emails across all pages. Gmail allows a maximum of 100 emails to be displayed per page, so with thousands of emails, there could be over a hundred pages! After some research, there is actually a little trick to help with this.