Easy questions are ones where you can just Google the answer. Hard ones are the ones where even after Googling you're still none the wiser. Here are some hard questions which I'd like to know the answer to:

Why are Plants Green

Sunlight is white - it is a mix of lots of different colours. But the maximum energy is in the green part of the spectrum. So why do plants reflect, and therefore not use, the biggest part of the sun's energy? How did that ever evolve?

Why do Purple and Violet look similar?

Violet is a pure spectral colour. That is, it consists of just one frequency of light - the highest frequency visible to the human eye. It's beyond blue in the spectrum.

Purple is a mixture of two different colours - red and blue. It looks bluish and reddish at the same time as you would expect.

But violet and purple look similar to the human eye. Violet has a "reddish" look, even though it is "more blue than blue".

Why?

This question is made more complicated by the fact that televisions and computer screens are not capable of producing violet light. They generally are capable of producing only three colours, red, green and blue (some use yellow as well), and everything else is made by mixing these three. The same is true of printing ink. The typical inkjet printer uses three colours, cyan, magenta and yellow (possibly with black).

You can't see real violet unless (1) you get it by splitting white light with a prism, or (2) unless there is a really unusual ink which produces actual violet.

Some additional information: the Birthday wrapping paper which looks purple to me looks blue to Izzy.

How did the P/Q division which splits Western Indo European languages divide Celtic down the middle?

Wikipedia, in the Gaulish entry, suggests that the k sound is older, and that it evolved into p in some Celtic languages. This doesn't, however, explain why this same change also happened in many other Indo European languages such as Germanic (where in some cases it went further and became f), Scandinavian, Greek etc.

What Language Did the Picts Speak?

Why did the people of Ireland adopt the Celtic Culture?

Some Hard Questions which I know the answer to

• Why are there no green stars?
• Why does a mirror reverse the left and right directions rather than the up and down directions?
• Why does the far side of the moon look completely different from the near side?
• Why is the moon exactly the same size in the sky as the sun?