Using a laptop as a second monitor

I need some help here... how can I actually use my laptop as a second monitor for my desktop PC? From what I've gathered while digging around online, you can't just plug an HDMI or VGA cable between them and expect it to work... which is pretty frustrating. Does anyone know what else I can do to make this happen? Thanks in advance!

Look, if your laptop only has video outputs—meaning it’s got those HDMI, DisplayPort, or DVI ports you already own—that just means it can send a signal to an external monitor or a TV. But if it actually has an *input*, then it can receive a picture from another device. So, I'll say it again: if there aren't any input ports available and you don't have the right cable, there isn't much I can do to help—you simply won't have that capability.