[Intro]   Welcome back to the fort, for more cool science about forts and forces!   Last time, we showed you how to build a sheet fort, using a bedsheet, some books, and a couple of chairs or just a nice, long piece of string.   Those forts were possible thanks to tension, the force that happens when we pull something tight.    By putting tension on the sheet, we were able to turn it into a roof for our fort and an indoor tent!   But tension is not the only force you can use to build a fort!    A force called compression comes in pretty handy, too.    While tension is the force of something being pulled tight, compression is the force that happens when things are pushed together.    Think of squeezing a spring in your hands. You squish it together by putting pressure on it. That’s compression.   The same thing happens when you put pressure on, say, a chair when you sit on it.    The legs of the chair get squeezed between the seat of the chair and the floor.    We can’t see the compression in the chair legs the way we do when we look at a spring being squeezed...but it’s there!    Now, what do you think would happen if the chair’s legs weren’t strong enough to handle all of that compression?   Well you might end up on the floor because the legs would break!   That’s why we make chairs out of metal and wood, instead of, say, raw spaghetti.    Although tasty, noodles wouldn’t be able to stand up to the compression that happens when you sit on the seat.   OK, but let’s get back to fort-building!   Now that we know what compression is, how can we use it to build a fort?   First, we need to pick materials that will be able to be stand up under pressure.   So, spaghetti is out of the question!   But what about those chairs that we used to build the sheet fort?    This time, try setting them up so the seats face each other, and leave some space between them.   Now just plop a sofa cushion on top of the seats. Even though we can’t see it, the legs of the chairs are getting compressed just a little bit. They’re strong enough to hold up the weight of the cushion so we have a nice little tunnel to lead into our fort.   But we can also use cushions —and compression — to build the walls of our fort.   This part is a two-person job, so you might want to get help from a parent, a brother or sister, or whoever you want to share your fort with.   You just need to stand up two cushions on their ends, so they’re far enough apart that you can get between them but still close enough together that another cushion can reach across the top, to make your roof.   With one person holding the walls up, the other can balance a third cushion on top, and, Ta-dah!    The wall cushions are compressed by the roof cushion. They’re being squeezed between the pillow on top and the floor.    Now, let’s inspect our fort before we go inside!   Now, if the pillows that make up your walls are too soft, then they might bend a little. That’s the compression pushing down on them and making them curve.    And if they’re really too squishy, then they won’t be able to stand up under the compression that the roof is putting on them. And in that case, your fort is just going to collapse.   That might be fun for a second, while all those pillows fall on you. But you’re going to have to come up with a better design.    Once you have a fort that you like -- and that can stay standing! -- you can try adding on to it, using the same ideas, to make it bigger.    Try adding a third wall, or, if you have enough pillows, try making your fort longer!   Now, your fort is good to go!    Squeaks and I love doing science experiments in our fort. What are you gonna do in yours?   Now you know compression is the force that we cause when we squeeze something, and we can use compression to build a fort. We just have to make sure that we choose materials that can hold up under the amount of pressure we put on it.     How do you like to make your forts? Do you have a picture of a fort that you’re really proud of?   Or do you have a question for SciShow Kids about, well, anything? Get an adult's help and let us know by leaving a comment or emailing us at kids@thescishow.com, and we'll see you next time.