For Halloween the last few years, we've decorated the house in a Harry Potter theme. This, of course, requires making many interesting props to entertain the Trick or Treaters. For example, the Animatronic Fawkes the Phoenix I made last year. One of the most popular props we've made is a Pensieve. As you may remember, the Pensieve is a vessel which stores memories. The memories can be viewed by sticking your head into the large misty bowl.

My Pensieve is made by creating a large glass bowl arrangement sitting over a laptop playing one of the Harry Potter movies. Pretty simple, but very effective.

Step 1: Materials Needed

Wood and Plywood-to build the enclosure the Pensieve will sit upon.
Plexiglas sheet
Spray Paint or other means of decoration
Glass Bowl (Pyrex is probably best as you'll be mixing warm water and dry ice).- Thrift stores are a great source of these. The fancier, the better.
Hot glue or clear silicone caulk
Laptop
One of the Harry Potter movies (I prefer the 1st one (Sorcerer's Stone), but the 3rd one is pretty good too.)
Plastic Bags
Water
Dry ice-can be found at some supermarkets, party supply stores (especially around Halloween) or ice companies

Step 2: Building the Enclosure and Attaching the Bowl

The basic structure of the Pensieve is a shallow box with a Plexiglas lid. The glass bowl sits on the Plexiglas, the laptop sits under the Plexiglas. The exact dimensions of the box will depend mainly on the size of the laptop you plan to use. For mine, the box is 3 in high x 15 in wide x 21 in deep. The box is open on one end to slip the laptop into. This is the side that faces away from the trick or treaters (unless you want to spoil the illusion!)

Once the box is constructed, you should cut a hole into the top of it. This hole should be larger than the size of the glass bowl, and will be covered by the Plexiglas sheet.

I used a 12 in x 12 in piece of 1/4 in thick Plexiglas to cover the hole, but again this should be sized to fit your bowl. If you're lucky enough to have a huge bowl, you'd likely want a larger piece of Plexiglas. To attach the Plexiglas, I ran a bead of silicone caulk around the hole in the plywood box top, and pressed the Plexiglas down onto it to make a good seal.

You now want to make the Plexiglas opaque, except for the area that will be covered by the glass bowl. I masked off the area that I wanted to remain transparent and spray painted the rest of the box and Plexiglas a light brown color. I had a rather fancy metal holder that the glass bowl sat in, so I didn't get fancy decorating the rest of the Pensieve. But if you want, this is where you can go crazy with decoration. You could also paint the sides of the bowl, as long as you keep the bottom of the bowl transparent.

The glass bowl now needs to be adhered to the Plexiglas sheet. Hot glue or clear silicone caulk can be used here. It's important that you center the bowl over the area of the Plexiglas you kept transparent.

Step 3: Laptop

When I first built this Pensieve, I was a bit nervous about putting a laptop directly under a bowl of water and dry ice, but I've had no problems over three Halloweens of use, so I'm a bit more relaxed about it now. Still, make sure you make things as water-tight as possible to avoid any potential laptop damage.

To do this, I make sure that there is a good seal between the plywood top and the Plexiglas. I also enclose the screen of the laptop (which sits right under the bowl) in a 2-gallon plastic bag. It's still transparent enough that it doesn't affect the viewing of the screen from above.

To start the Penseive, insert the DVD of your choice into the laptop, and lay out the laptop so it's flat (180 degrees). Now, if you think about it, a Trick-or-Treater facing you looking down into the Pensieve will see the movie playing, but it will be upside down! A semi-secret PC command (Control+Alt+down arrow) will flip the screen so it'll look upside down to you, but right side up to someone facing you. (This command is also great fun at work on April Fool's Day!) (Control+Alt+up arrow puts it back normal.)
Slide the laptop into the box so that the laptop screen is positioned right under the glass bowl.

Note to Mac users: Flipping the screen image upside down seems to be possible with a Mac as well but a bit more complicated, at least with my iMac--don't know if it's the same with Macbooks. Open System Prefs. Hold down Option+Command and click the Displays icon. When the window opens, you'll have a new drop down menu called "Rotation" that you can use to flip the display. Note that the mouse movement DOESN'T flip, so you have to do some interesting hand/eye coordination. I don't know how this affects the system, so use at your own risk.

Step 4: In Operation!

To complete the Pensieve, all you need is warm water and dry ice. Add some water to the bowl and throw in some small pieces of dry ice so you get a nice bubbly foggy effect. Because of the viewing angles needed with the laptop, the viewer really has to get right up on top of the fog to see the movie through the mist below.

Below is a photo of what the view looks like without water or dry ice in the bowl (sorry, I don't have a photo of what it looks like filled--just imagine a view with swirling mist in front of it). Also shown are a couple of pictures of the Pensieve in operation during Halloween.

Hope this Instructable inspires you!