Answers to Do You Know? on Page XX
1. Flies are difficult to swat because they are so much quicker than us. But can't we move so slowly that a fly can't detect the movement and actually get it? 
Ans: Flies process motion information very fast, and so you are right. In principle, you can trick the fly by just moving very slowly. How fast the edges of your hand expand relative to the fly's vision is what triggers the fly to flee, so a slow hand could confuse the fly. 
But then the fly motion vision is so sensitive that you would have to move so slowly that either you get bored and give up, or the fly just takes off for greener pastures. That is another characteristic of flies, they do not stay in one place for very long.
Apparently, confusing the fly with a clap of the hands makes the job easier. You approach the fly at normal speed and, instead of slapping it, clap your hand just above the fly to intercept it as it takes off. Some scientists have even worked out equations describing this!
You could also hold perfectly still and watch the fly until it starts washing itself, then strike quickly while it is distracted, just like jumping on someone in the shower. 
One other solution is to spread your fingers wide on the same surface as the fly and pull your middle finger back like a slingshot while you slowly slide your hand towards the fly and quickly release the cocked finger once the fly is in range. Since the fly is being approached from multiple directions it apparently gets confused enough to not take off as it normally would to an approaching hand, so you can effectively crush it when you release your middle finger. 
Poor fly! If flies could know how much fun scientists are having plotting their capture ....

2. Genetically, who are we closest to -- our parents, our chidren, or our siblings?
Now this is a really interesting question. When you ask children this question, they typically say parents, and when you ask adults, they think it is obviously their children.
The short answer is that it is a little bit complicated, as is usually the case in science. You are 50% related to your parents and you are 50% related to your children. When it comes to siblings you are 50% related to them *on average*. In particular, we get half our DNA from our mother and half from our father, so we share our genetic material equally with our parents.
The reason is that when you inherit DNA from your parents, you get each of your 23 pairs of chromosomes from one parent. However, there is some mixing. If you comparing different siblings (brothers and sisters) they could take different parts of that chromosome and so it is possible for you to be taking the same parts or completely different parts. That is why it is 50% on average.
Of the 23 pairs, the last one is the sex chromosome. Females have XX sec chromosome pair while males have XY. All children inherit an X chromosome from their mother; girls inherit X from their father while boys inherit the Y chromosome (yes, it is the father who determines the sex of the child!). The sex chromosome (and the presence of mitochondrial DNA) complicates the issue. All daughters inherit the same X chromosome from their fathers, while they may inherit either of the X chromosomes of their mother. Hence on the average, sisters share 75% of this genetic material, while brother and sister share only 25%. There are always small differences because of a process called recombination in which some amount of genetic mixing happens, so we are all actually unique. This is what DNA fingerprinting is based on. Identical twins share their entire DNA exactly.

3. How do electric eels get their energy?
Perhaps they have built in generators? The answer is that electric eels use modified muscle cells. They are called electrocytes and they are strung together (like on a necklace) along the inside of the fish. If you connected a battery end to end, and you connected the plus of one battery to the minus of another the voltages add together. The same things happen to the fish too. These electrocyte cells slowly accumulate voltage and over a very big eel you maybe get 500 volts and they are discharged from just below the chin of the eel and its tail. (Note that eels live in water, which provides additional outlets for the current. So though generate a large voltage, the current generated is divided, and therefore small.)
So yes, the eel is basically a giant accumulator or battery. Tt is pumping ions of sodium and potassium in and out of these cells to generate those tiny voltages which together, over the course of these thousands of cells, add up to make hundreds of volts.
The eel has a highly specialized nervous system that has can synchronize the activity of these electrocytes. The nervous system does this through a command nucleus that decides when the electric organ will fire. When the command is given, a complex array of nerves makes sure that the thousands of cells activate at once, no matter how far they are from the command nucleus. 
Why don't electric eels shock each other? Apparently scientists are not entirely clear why they can shock other animals without shocking themselves. One possible explanation is that the severity of an electric shock depends on the amount and duration of the current flowing through any given area of the body. For the purposes of comparison, an eel's body has roughly the same dimensions as an adult man's arm. To cause an arm to spasm, 200 milliamps of current must be flowing into it for 50 milliseconds. An eel generates much less energy than that because its current flows for only 2 milliseconds. Additionally, a large part of the current dissipates into the water through the skin. This probably reduces the current even more near the central nervous system or heart. A prey 10 times smaller in length than an eel is about 1,000 times smaller in volume. Therefore, the small animals close to the eel get shocked, rather than the discharging eel itself.

4. Why is there such a big `gap' between Mars and Jupiter? There is an asteroid belt, but why did no planets form there?
Scientists seem to consider that it was Jupiter that stopped any planet growing where the asteroids now are. 
There clearly was planet-forming material all the way from near the Sun outwards to Jupiter and beyond. This should have clumped together into larger and larger bodies, eventually planet-sized bodies. But close to Jupiter, its gravity would stir up the orbits of such bodies just inside its orbit, which is where the asteroid belt is. This would cause collisions between such bodies to be too energetic to allow materials to clump together. So very large bodies cannot grew. 
The largest bodies there today are less than about 1,000 kilometres across. So they are stirred up and most of the material has, in fact, been lost because of Jupiter. We could not make a planet now if we stuck all the asteroids together. They would make only about a hundredth of an Earth mass in total. This is a surprisingly small amount even though there are thousands of bodies in the belt.
Where has all that material gone, then? Have they become asteroids that rained down on us in the early phases of the solar system? Most of the material has been scattered out of the solar system. Some material scattered inwards and would have hit Mars and the Earth and Venus and Mercury, and doubtless, some has been gobbled up by Jupiter. Now Jupiter 318 times the mass of the Earth, so this would scarcely be noticeable.
So yes, the verdict is clear: Jupiter is the culprit!

Source: Scientific American and Cambridge Science for Kids.