A. Because the _carbon_ of the leaves is a _bluish olive_, and the _sap_ and _tissue of the cells_, YELLOW; when, therefore, the _yellow sap_ flows into the _blue carbon_, it produces a _green leaf_. Q. _Why are leaves a LIGHT green in SPRING?_ A. Because the young leaves of spring have _more sap_ than _carbon_; and, therefore, the _yellow_ of the green prevails. Q. _Why are leaves a YELLOWISH BROWN in AUTUMN?_ A. Because the _carbon_ of the leaves is _dying away_, and the yellow tinge of the _tissue_ and _falling sap_ prevails over the blue. Q. _Why are plants a PALE YELLOW when kept in the DARK?_ A. Solar light is essential for the production of _carbon_; and as plants kept in the dark _lose their carbon_, they lose the _blue colour_ which should convert their yellow sap to _green_. Q. _Why are POTATOES YELLOW?_ A. Potatoes are grown _underground_, and, therefore, contain very little _carbon_ (or blue colour); hence the _yellow sap_ of the potato is not converted to green by carbon. Q. _Why are potatoes (which grow EXPOSED to the air and light) GREEN?_ A. Because the sun-light _increases their carbon_; which (mingling with the yellow sap) turns the potato _green_. Q. _Why is it DANGEROUS to SLEEP in a room which contains LIVING PLANTS?_ A. Because they _exhale carbon in the dark_ in the form of CARBONIC ACID GAS, which is destructive to animal life. Q. _Why are SOME things (like glass) TRANSPARENT?_ A. In transparent bodies (like glass) all the rays of light _emerge on the opposite side_. Q. _Why are SOME things SHINING and splendid?_ A. Those objects which _reflect the most rays_ are the most _splendid_; and those which _absorb_ them most, are _dull_. Q. _Why are DESERTS so DAZZLING in summer time?_ A. Because each separate grain of sand _reflects the rays of the sun_ like a mirror. Q. _If you move a stick (burnt at one end) ROUND pretty briskly, it seems to make a CIRCLE OF FIRE,--WHY is this?_ A. Because the eye _retains the image_ of any bright object, _after the object itself is withdrawn_; and as the spark of the stick returns _before the image has faded_ from the eye, therefore, it seems to form a _complete circle_. Q. _If separate figures (as a man and a horse) be drawn on separate sides of a card, and the card TWISTED quickly, the man seems to be seated on the horse,--WHY is this?_ A. Because the image of the horse _remains upon the eye_ till the _man_ appears. The Thaumatrope is constructed on this principle. Q. _Why do the STARS TWINKLE?_ A. Fixed stars are _so far off_, that their rays of light do not strike upon the eye _in a continuous flow_, but at _intervals_: when their rays _reach the eye_, the star becomes _visible_, and then is obscured _till the next batch of rays arrive_; and this _perpetually_ occurring, makes a kind of _twinkling_. Q. _If we look at a RED-hot FIRE for a few minutes, WHY does every thing seem TINGED with a BLUISH GREEN colour?_