Finding the Heavy Coin by weighing twice Suppose you have $100$ coins. $96$ of them are heavy and $4$ of them are light.  Nothing is known regarding the proportion of their weights. You want to find at least one genuine (heavy) coin. You are allowed to use a weight balance twice. How do you find it?
Assumptions: 
Heavy coins all have the same weight; same for the light coins.
The weight balance compares the weight of two sides on the balance instead of giving numerical measurement of weights.

To avoid wasting readers' time when posting puzzles, it is useful (and will improve the upvote/downvote ratio) to provide:


*

*an indication of the difficulty level. Was this puzzle in a math-is-fun book for children, or is it a generalization of a problem from the final round of a national olympiad?  Having schoolchildren attempt a question that (they were not told) is the $n=100$ specialization of a combinatorial lemma from a research paper, is a waste of time.  Having mathematicians spend time on (what might look like) a coin weighing puzzle from the literature, but is really an easy task the high school students could solve with some patience, is a waste of resources.

*the source, if known.  This helps those who want to evaluate the difficulty before taking time to attempt a solution, or who would like to look up hints or answers.

*indicators of whether the problem is correct or solvable as stated.  Problems from a book, a competition or a journal are likelier to be correct and unambiguously formulated.  Questions invented, circulated and (believed to be) solved among friends or students are more likely to have pitfalls that were missed in setting or solving the problem.

*clear definitions.  In this case, coin weighing problems involve at least two very different interpretations of "use a weight balance".  Weighing can mean comparison or numerical measurement and what is possible is completely different in those settings.
Not specifying which is meant can waste the time of users who try to solve the problem in the wrong interpretation. 
