Forced perspective is a technique that most people are aware of but may not necessarily know what it's actually called. Forced perspective is where you try to line up your subject or subjects with other objects in your frame to try to force a certain perspective. You would have seen this type technique in every photo of the Leaning Tower of Pisa where people line up the photos to try and make it look like they're holding up the tower.
18mm 1 /15s F3.5 ISO 3200 - Leaning Tower of Pisa, Pisa, Italy
For this photograph you can see that all you have to do is line up your subjects and find the right angle so that they interact with each other in the way that you want. In this case of course, we're trying to make it look like she's holding up the tower.
There are many examples online of where people have used this technique to achieve a lot of interesting and hilarious photographs. My personal favourites are the ones where people turn the camera sideways to make it look like people are walking on the floor or standing over a cliff. Unfortunately I haven't quite achieved one of these pies myself so I'll just show these ones that I've done.
18mm 1 /640s F2.8 ISO 100
For this one, there was giant tap display so I got my friend to stand a few metres in front of it and tried to position my view so that it looked like it was draining straight
My friend just stood in front of the computer display and I just positioned myself to make it look like he was typing on it.
35mm 1/60s F2.5 ISO 100
For this photo, the small girl was just standing further along the path. I think it would've looked better if there was no path but alas I didn't get a chance to try. I wanted to use a small aperture to get a large depth of field so that both people could be in focus.
29mm 1/50s F1 .8 ISO 100
In this photo, my friend is actually crouching behind the other and I positioned my view just so it would like the other girl is holding her severed head.
29mm 1/SOs F1 .8 ISO 100
So these are just some examples of what you can do with forced perspective. It's an incredibly fun technique and something you can really get your friends involved with. Give it a shot and see what you can come up with.