用英文回答..thx

Hope that can help u

1)為什麼好蛋會沉在水中而壞蛋會浮起來?
Why good eggs sink in water while rotten eggs float on it?
There is a small air pocket in the large end of the egg. When the egg is fresh, the pocket is only about 1/8th of an inch deep and as large around as a dime. As the egg ages, however, it loses both moisture and carbon dioxide — shrinking — so that the size of the air space increases. And the size of the air space determines the buoyancy of the egg.
So if you submerge a very fresh egg in water, it will lie on the bottom. An egg that is a week or so old will lie on the bottom but bob slightly. An egg that is three weeks or so old will balance on its small end, with the large end reaching for the sky. And a bad egg will float.

http://www.ochef.com/789.htm

2)水與油為什麼不能混在一起?
Why oil cannot be mixed with water?

The following link explain the process with pictures in a very easy understand way
http://www.newscientist.com/article.ns?id=dn3408