A few months ago, Caleb became fascinated with how things can combine to make other things. Tim told him about hydrogen and oxygen making water, and Caleb thought the whole concept of "elements" was cool.
So today he started asking more about it, and we decided to introduce him to the periodic table of elements.
There are a lot of college-level tables of elements available online. But we did find some that are kid-friendly. This site: http://elements.wlonk.com/ has the very best periodic tables for kids I found. Not so oversimplified that they're useless; each square includes the NAME of the element, not just the symbol (rare, suprisingly); there are pictures and words showing where a kid might encounter something that uses that element (like Arsenic is in LEDs), and bunches of interesting information. All in a PDF that you can download and print for free. Go look. Very cool.
We also found http://www.starfall.com/n/N-info/download.htm , where you can download reading and writing worksheets free. Their reading philosophy is like the "Bob Books" philosophy, so my new readers love it. They can actually read a whole book--with a plot--by themselves.
http://people.howstuffworks.com/index.htm is for the perpetually curious about stuff. It's not just mechanical--they cover stuff like the CIA, and "love".
And http://www.intute.ac.uk/timeline2.html is one of the better, more accessible timelines of world events, for those who just want to know what was going on in the world at the same time as the British first ate bananas. It is a UK site, so the American experience is somewhat slighted. This is not a problem before 1630 anyway. Very informative.
So there you go. Some of our favorite resources for answering kid's questions online.
Of course, the first thing we usually do for specific questions (like "What is pepper made from?") is go to Wikipedia. Then Google it. Those are great sources, direct, and include pictures.