An infinite two-dimensional pattern is indicated above.
The smallest closed figure made by the lines is called a unit triangle. Within every unit triangle, there is a mouse. At every vertex there is a "sweet". What is the average number of "sweets" per mouse?
The answer is obviously 1.
Since the pattern is infinite, there's a countably infinite number of mice and a countably infinite number of sweets. Thus there's a bijection between the sets of mice and the sets of sweets, hence there's exactly one sweet per mouse and one mouse per sweet.
Wait a moment... I made a mistake. I forgot to take into account the insatiable hunger these mice have for sweets - each single mice eats all the sweets which lie on a single line. Luckily, since there are countably infinitly many parallel lines, each mice gets its own full line to eat, thus there are actually are countably infinitely many sweets per mice.
I do hope that after eating all those sweets, one of the mice will have the decency to eat this exercise too...