This is a super little make for those with access to a 3D printing service.
Step 1: Materials
A glass jar, the more "laboratory glassware" looking the better.
These can be found in most "Bed & Bath" shops; I got this one from Wilkinson (UK) for a couple of pounds. Charity shops are a great source of glassware too BTW! Your jar doesn't need a rubber seal as it doesn't need to be airtight. Jam jars work too.
A substance called "Gelli Baff" which is manufactured here in the UK, not sure of product availability elsewhere, try Amazon. It comes in powder form; Powder 1 makes tap water gloopy and coloured (great for kids' lucky dip), powder 2 makes it thin enough again to go down drains/sewers. I've only seen this in red or green. The sachets will thicken a large body of water, use sparingly, trust me on this ;)
3D printed brains, I used this Thing
scaled to about 40%
Your project will require enough brains for a single skin on the inside of your jar; One of the reasons I used Gelli Baff was it's so highly viscous it holds the brains in place and the viewer of your finished piece doesn't get to see the back side of the bottle-opener brain Thing, plus the project required less brains to "fill" the jar.
Step 2: Making Up the Gelli Baff
Fill your jar with cold tap water to desired level.
Sprinkle the powder (sachet 1) into the water and stir, the water will colour and thicken.
Resist the urge to use a lot of powder in one go; It's a lot easier to "go thicker" in stages!
Step 3: Placing Brains in Jar
You might want to wear disposable vinyl gloves for this bit!
Simply because it's messy, the Gelli Baff substance itself is non-toxic.
Place a single skin of brains against the inside of the glass top-side showing out, to form the bottom layer.
Continue to build up the layers. When you're done use a piece of kitchen towel or some disposable rag to clean up the inside and rim of the jar.
Replace the lid and you're done!