Don't pour it down the drains when you are finished, though!

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Storing a universal solvent
You can store universal solvent in a glass jar, as long as you have already added enough powdered glass to saturate the solvent with glass.

If you need to have the universal solvent work on glass, you would need to use the iron-saturated stuff that you keep in an iron vat instead.

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How high is that building?
The standard ("guess what I'm thinking") answer is to use the atmospheric pressure at the bottom and the top, and from this, deduce the actual height of the building.

The interesting answers I have heard include, in no particular order:

• giving the barometer to the building manager in exchange for the information you need;
• dropping the barometer from the top of the building and timing its fall (rather enivironmentally undesirable!!);
• using the barometer as the weight of a pendulum, so you can dangle it on a string and time the period of the pendulum;
• lowering the same pendulum, pulling it back up and measuring the string;
• measuring the shadows of the pendulum and the building and using thshadow ratio to get the height ratio;
• using the 800 mm barometer as a yardstick, working your way up the fire stairs, marking off 800 mm rises as you go.

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Sorting the eggs
Put one of the eggs on a dinner plate, and spin it fast. Then stop it, and watch what happens. Then do the same with the other egg, stopping it as it is spinning fast, and letting it go again.

The difference you see is due to momentum: think about what a liquid can do inside and egg shell that a solid cannot do. If you think about it, you may be able to relate this to why we feel giddy after spinning around -- look up "semi-circular canals" to find out more.

Now here is a puzzle I don't have an answer for: maybe you can find one: two cans of drink sit on the shelf in the refrigeratot. You know that your mischievous friend has just shaken one of them, very hard. How can you tell which is which, before you open it? (I would take the left-hand one, tell the friend it is the left-hand one, point it at the friend, and go to open it -- if the friend ducks, I would take the other can, and repeat the experiment, but this time, I would open it, anyhow :-) Which is well and good, but how can you telll if the friend isn't there?

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Lens under water
The power of a lens comes mainly from the difference between the refractive index of the lens material and the refractive index of the surroundings. Air has a refractive index of about 1, water is about 1.33, glass is about 1.5 -- work it out!

The answer annoys me -- as I get older, I see less and less when I go snorkelling, and I recently had the bright idea of attaching an old pair of spectacles to the outside of my mask. A few moments thinking, though, and I realised that I would not get the result I wanted. Can you work out how I could get the necessary help? (Hint: pince-nez .)

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A round trip of the moon
Six fuel containers carry you one-sixth of the way around the moon. With a full load of fuel, you can travel out one sixth of the way, dump twelve containers, and return. On your second trip, refill your tanks, and drive out to one-third of the way, dump twelve containers, return to the staging point, load the remaining six containers, and return to the base camp. Then load up with a full load of fuel, and travel around the moon in the opposite direction, arriving at the one-third dump, just as you use up all your fuel.

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Can you make this shape?

All I am going to say is that it can be done. :-)

In fact, I once made one from a sheet of copper -- it used to sit on my desk and puzzle people who assumed I had used solder in the making. You will have to take my word for it that I hadn't.

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