Example #1 Even, Odd, and Total Digits
 Start with any number, say 9288759
 Count the number of even digits,
the number of odd digits,
the total number of digits.
 You get 347
 Repeat and you get 123
 Repeat and you get 123
 Repeat and you get 123
 Once you reach 123, you never get out,
just as reaching a black hole in physics implies no escape.


Example #2 Words to Numerals (from Martin Gardner)
 Start with any whole number, and write out its numeral in English.

 Start with the number 5 > FIVE
 Count the number of characters in its spelling (count spaces and hyphens).
 4 > FOUR
 Repeat and you get 4.
 Repeat and you get 4.
 Once you reach 4, you never get out, so you have reached the black hole.

 Start with the number 163 > ONEHUNDRED SIXTYTHREE
 Count the number of characters in its spelling (count spaces and hyphens).
 23 > 12 > 6 > 3 > 5 > 4
 Repeat and you get 4.
 Repeat and you get 4.
 Once you reach 4, you never get out, so you have reached the black hole.

Example #3 Sum of the digits of the divisors
 Start with any number greater than one,
and write down all its divisors, including 1 and itself.
 Now take the sum of the digits of these divisors (each individual digit!)
 Eventually, you get 15, the black hole.
 For example, start with 20.
 The divisors are 1, 2, 4, 5, 10, and 20.
 Adding the digits, you get 15.
 The divisors are 1, 3, 5, and 15.
The sum is 15, which repeats.
