Structures in Roblox do not have to be static! The Roblox physics engine allows for many types of movable structures and contraptions using joints. A very simple, yet useful, contraption to make is a door.
Start by making the frame of the door. Use the Part tool to insert four parts, one for each part of the frame. Use the Scale tool to resize the parts so that they are the size you want for your frame.
Next, we need parts to hold the hinges for the door. A hinge is a special type of joint in Roblox that only allows rotation. Create a 1x1x1 part and place it on the bottom of the frame. Make sure it is one stud away from the edge of the frame:
Now click on the Surface tool and select Hinge. Then click on the top of the 1x1x1 part.
A small axle will appear on the top of the part.
Now we'll make a hinge for the top of the door. Make another 1x1x1 part and place it at the top of the door, directly above the first 1x1x1 part. Use the Surface tool again to put a hinge on the bottom of this new part.
Now we need to make the door itself. Create a new part and scale it so that it fits in the frame. Make sure that it doesn't fully fill the frame, otherwise the door may not open.
Try playing your game to see the door swing!
The gaps in the door are needed to let the door swing open, but they do not have to be that wide. By default using the tools such as Move and Scale only work in 1 stud increments. We can work in a smaller increment by clicking the 1/5th Stud radio button. This will let us drag or scale parts .2 studs at a time instead of 1. Click on the 1/5th Stud radio button and use the Scale tool to stretch the left and right sides of the door frame closer to the door. Be sure to still leave a little gap on either side.
Now you can use the Scale tool to make the part thinner.
Make the top hinge part thinner.
Now use the Scale tool to make sure the main part of the door touches both the top and bottom hinge parts.
Now the door looks much better and will still swing open. If you want to go further, try changing the Color and Material of the door and the frame.