The candle problem

If you are an experimentalist, please just skip it. I'll post something for you tomorrow.

Here is a famous test by Karl Duncker, he created it to study the "functional fixedness". So, you have a candle, a box of thumb-tacks and a book of matches:



The task is to attach the candle to the wall, in such a way that the wax doesn't drip onto the table (or the wall).

This is not kind of complicated, or is it? However the results of Duncker's studies were amazing - people solving the problem just to make fun were much faster than those, who were promised a reward (say, 50 dollars) for being the fastest.

The greatest math problem ever. Part 2.

As a comment to one of my posts, Dave proposed the following puzzle:

1
11
21
1211
111221

what's the next line?

Project Eureka

Robert Benea left a comment to "The greatest math problem ever" post, with a link to the Project Eureka website, featuring a huge number of different math and logical puzzles. He also added the problem there, in the "very easy" section.

The greatest math problem ever

This is a problem, which can be easily solved by children before entering elementary school. If you want to give it a try, please forget everything you have ever studied. Here it comes:

8809 = 6
7111 = 0
2172 = 0
6666 = 4
1111 = 0
3213 = 0
7662 = 2
9312 = 1
0000 = 4
2222 = 0
3333 = 0
5555 = 0
8193 = 3
8096 = 5
7777 = 0
9999 = 4
7756 = 1
6855 = 3
9881 = 5
5531 = 0

2581 = ?

I took it from here (Russian)