These enigmas are all in the form of some sort of question which can be formulated in different ways. So far we have identified and categorized the following.

How Much (How Many) ?
The answer here is always numerical,
     not always a whole number, but some form of value,
          for people, objects, times, speed, time, distance ...
Mary's Siblings, The Frog, The Bath, Apples, Restaurants, The Worm, Diagonals

How Much and How (Why, Where) ?
The answer here is also numerical,
     but to find it one must know how the process works,
          and the nature of this "How" is sometimes a question of "Why", sometimes of "Where".
The enigmas of this category are tricky, and in general more difficult
     than those in the preceding How Much category.
The Chain, Weighing

How To ?
The answer here is more elaborate,
     the description of a process which achieves the desired result.
          The degree of "difficulty" is not easy to establish here,
               the answer being often quite simple but not evident,
                     the "Why didn't I think of that" kind of thing, great for children.
River Crossing, Finding The Faulty Coin, Three Lights, Calendar

What and How (Why) ?
What is really happening in this mysterious event
          and How does it work, or Why is it this way ?
     Sometimes numbers are involved and sometimes not.
Sequences, Hats, The Hidden Pea

What Happened ?
These enigmas are rather weird.
     Usually in the form of a story which comes to an impossible end,
          the job here being to find out what went wrong.
     We had only one for quite a while, but the three lodgers now have algebraic company.
The Missing Dollar, 2 = 1