You mean the statistics demonstration? I'm sure I've seen it in several places.

I know of two tricks for detecting the students. The first is to look for six or seven heads or tails in a row. Over a hundred tosses, a coin will probably do that, but humans "being random" won't. The other is to look at the page as a sequence of "HHH" and "TT" strings and estimate how many there are. A coin, of course, changes from heads to tails 50% of the time, but a human does it more like 70% of the time.

I'm sure there are other characteristics, too, but those two are sufficient to throw out most human attempts at a glance. It's actually kind of obvious, when you see the two side by side.

Me "being random" with the numpad: 10110101001010010101011010100101011010100101010110101

Computer-generated random: 1101110100000000011111110111011000010001010110011111

See? Here's a few more. Try it.

1000100111011001010000001001010110111000011011101011

1101010001010010101110100101000101010111101001001000

1101111010110111100010110001000100001001111001001110

1100000010000010101001000001101001101011111100111001

Cool! Makes sense, too— it feels unrandom to sit there hitting one key a bunch. How do you know when to stop?

So here's what I got: figure I have a bias to switch keys. That means that 01 and 10 are more common than 11 and 00. So what I need to do is group 01 with 00, and 10 with 11. What I do is generate twice as many bits as I need, treat the string as a sequence of two-bit pairs, and reduce each one to its first bit. That looks like this: 01011110000010111000010011111011111111000000111010110001111000111100111100001100011010011101

Gets you past the litmus test, but looks like it goes too far the other way? Hard for me to tell, actually.

Another thing would be something like a sequential xor of each bit in triples (i.e. 010 -> (0 ^ 1) ^ 0 = 1), which segments triples across probability like so:

     -       +           +       -
    000 001 010 011 100 101 110 111
      0   1   1   0   1   0   0   1
   

You can do that quickly by counting the 1s— 1 or 3 is 1, 0 or 2 is 0. That looks like this: 00011011011011000000111111001011000101010000100100110011011010

I don't know if it adds much more (apparent) entropy, though.

Data:

    011101101010100100010100100010111001010100110000101011101101110
     0 1 0 1 1 1 1 0 0 0 0 0 1 0 1 1 1 0 0 0 0 1 0 0 1 1 1 1 1 0 1
      0  0  0  1  1  0  1  1  0  1  1  0  1  1  0  0  0  0  0  0  1

    111010111010100101000101011110100011011011010001111011110101001
     1 1 1 1 1 1 1 0 0 0 0 0 0 1 1 1 0 1 0 1 1 0 0 0 1 1 1 1 0 0 0
      1  1  1  1  1  0  0  1  0  1  1  0  0  0  1  0  1  0  1  0  0

    101110100101101010110101010001101101010010110111010110101001110
     1 1 1 1 0 0 1 1 1 1 0 0 0 0 0 1 1 0 0 0 1 1 0 1 0 0 1 1 1 0 1
      0  0  1  0  0  1  0  0  1  1  0  0  1  1  0  1  1  0  0  1  0