brainteasers

We could do the same with colors, btw and would have a different result. I think the main question here is that there are no other colors then those of the background and foreground. The little squares which appear to be between those two colors are an optical illusion.
--> this will always be the case, except for those which are blind

The google explanation is a bit flawed.
Because colors dont have just one wavelength, they have a spectrum of wavelengths.
e.g. The color red has a spectrum of 620–750 nm. Inside this spectrum the color can have mutiple gradients.

The color white contains all other colors therefore its spectrum is 380–750 nm.
So if a spectrum is enough to definy the color red, why shouldnt be a somewhat larger spectrum be able to definy the color white?

The google definition compares apples to oranges.

Agreed OP apples with oranges

So we have one colour white
And then there is black

I wonder how an artist thinks about it black&white can create some powerful images.

When I first looked at the brainteaser I thought both were colours

i would say 1 wavelength is exactly 1 color. White is a mixture of wavelengths that contains red, blue, green, yellow colored wavelengths.

nope
red exists at a wavelength of 620 nm
it also exists at a wavelenth of 750 nm
and it exists at a very high amount of wavelegths between these values.

yes, red in general, but not a specific red (either its more purple or more orange...), there are different reds, and you have one red at 640 nm and a different one with 700 nm

but it is red....

if you say that its a different red then you go by my physical explanation which would mean that there is in incredible high number of colors

x+x+x=30
2x+y=40
x+2y+2z=60

x=10
y=20
z=5

Two pupils - a boy and a girl - have just taken meteorological measurements.
Now they are having a rest on a hill and looking at a cargo train passing by.
The steam locomotive is puffing and smoking desperately heading uphill. A steady wind, without blasts, is blowing along the railway.

"What wind speed have our measurements shown?" - the boy asked.
"7 meters per second".
"Today this is enough for me to determine at which speed the train goes."
"Is that so?" said the girl in doubt.
"Just look carefully how the train moves".

The girl reflected for a moment and then understood what the boy meant.
The pupils saw exactly this picture captured by a painter:

So at what speed did the train go?

Correct! Trains were slow at that time

A boy has as many sisters as brothers; his sister has two times fewer sisters than brothers.
So how many sisters and brothers are there in the family?

Quote from denne

2 boys and 1 sister.

In this case the boys' sister would be the only girl in the family, and thus have no sisters, which is not right. Try again
(Two times fewer means "0.5x", not "x-2".)

Correct
In Russian one can say it without any ambiguity, seems I just don't know the right words in English

Okay, here is another one:

This man needs to ferry a wolf, a goat and a cabbage head in a boat across the river.
The boat can only take a man and either a wolf, or a goat, or a cabbage head with him. If the wolf and the goat are left alone on one bank, the wolf will eat the goat; if the goat and cabbage head are left alone, the goat will eat the cabbage. But in man's presence nobody eats anything.
The man actually managed to ferry everything to the other bank of the river. How did he do that?

Correct!
There are also other ways, e.g. this one:

Next:

Wandering around the world, I have been to such places where the difference of temperatures between night and day was so great that my watch changed its clock speed when I spent days and nights on open air. I noticed that temperature change during day caused the watch to become ½ minutes fast, and during nights the watch became ⅓ minutes slow. In the morning of October 1st the watch was still showing the correct time. By what date will the watch be 5 minutes fast?

I guess the above task required too many calculations and was more of a time killer than a brainteaser... I already forgot the answer anyway...

Here is a quick one:

Five apples lie inside a bin. How can one share these apples between five girls, so that each girl gets an apple, and one apple remains in the bin?