Sheldon, Leonard, Howard, and Raj create an oobleck using corn starch and water. Initially, it's a liquid, but when they turn the music on, it resembles a solid. Sheldon explains the phenomenon to Penny: it's a non-Newtonian fluid which is liquid but becomes solid due to the percussion from the speaker.
Beakman demonstrates that marshmallows are less dense than the same amount of water and therefore, marshmallows float. Nails are heavier than the same amount of water so they sink. He then explains water displacement by getting into a tank of water. His DIY demo for this is making a aluminium boat that floats in a bowl of water but when you crush the aluminum foil into a square, it sinks because it couldnt' displace the water.
Beakman explains that construction equipment is "strong" because of hydraulics. He demonstrates that when you try to compress a liquid, it has a limit. Pascal's principle says that the pressure is the same everywhere in the liquid so if you open the end of the syringe, then the liquid is forced out. Using this, they show how Liza can push down on one hydraulic shaft and this will cause the rat to be lifted. Basically, she does more work with less energy.