What it shows
The bulk modulus of water is about 2.2 x 109 Pa, which means that a change of 1 N/m2 of external pressure on the liquid is able to change a given volume of it by a factor of 4.5 x 10-10 (for comparison, the same pressure change would produce a volume change of about 7 x 10-6 for air and 7 x 10-12 for cast steel ). So if we can completely fill a Florence flask with water, we can use it as a hammer to drive a nail into a board!
How it works
Instead of an empty glass flask hitting a nail—an interaction that would easily exceed the elastic limit of the glass and cause the flask to shatter—the water-filled flask is effectively a solid mass, with the water preventing the glass surface from deforming. The liquid core also has the ability to distribute the shock isotropically throughout the spherical part of the flask, allowing the entire surface of the glass to absorb the impact.
Setting it up
As mentioned above in the "What it shows" section, we want the flask to be completely filled with water. Nitrogen and oxygen gas from the atmosphere can dissolve in water fairly easily. Since gases are a few orders of magnitude more compressible than water, the presence of a small air bubble inside the flask can significantly reduce its overall strength and increase the risk of the flask shattering in the demonstrator's hand.
Fill a 1-liter Florence flask with distilled water. To do this properly, boil the distilled water for several minutes in a seperate beaker and let it come back to room temperature before filling the flask. (Even better, pump on a flask of room temperature water using a filter pump or aspirator while the flask is submerged in an ultrasonic cleaning bath, until no more bubbles appear.) Seal with a rubber stopper and silicon rubber sealant. Use a thin wire wedged between the stopper and the neck of the flask to allow excess air to escape as the stopper is pushed in. Electrician's (plastic) tape can be used to ensure that the rubber stopper remains in place.
With this "water hammer" you can strike a nail into a piece of wood using reasonable force. For the target, use a short piece of 2 × 4 with a large flat head nail already knocked some way in so that it stands freely.
When using this tool, be careful to tap the nail squarely on the head—a side impact will likely shatter the flask. Wear goggles and thick leather gloves. Be mindful that just below the soft board is a very hard lecture bench.