Core Mechanics - Fundamentals of Game Design - page 303

of fish that may be in a river at any one time, so the river stops producing fish
when it gets full.
Sources can be global mechanics: A mechanism that pays the player interest at reg-
ular intervals on the money he owns would be one example. An interest-payment
mechanism applies throughout the game regardless of anything else, so it is global.
Sources can also make up part of the mechanics governing the behavior of entities.
In The Settlers, a stream that produces fish is an entity, one of whose attributes is
the number of fish it contains.
Sources can be limited or unlimited. In Monopoly , the “Go” square constitutes
an unlimited source—according to the rules, it can never run out of money. (If
the bank runs low, the banker may make more money by writing on paper.) But
the collection of houses and hotels stored in the bank is a limited source: Once
the banker sells all the houses and hotels, no more may come into the game. The
stream in The Settlers is an unlimited source of fish. Although it can be temporarily
empty if too many fishermen are catching fish from it, it goes on producing fish
until it is full, as long as the game is running.
Drains
A drain is a mechanic that determines the consumption of resources—that is, a rule
specifying how resources permanently drop out of the game (not to be confused
with a converter , which we'll look at next). In a shooter game, the player firing his
weapon drains ammunition—that's what makes ammunition, a resource, disap-
pear. Being hit by an enemy shot drains health points. Enemies drain out of the
game by dying when their health points reach zero. The most common drain in a
construction and management simulation is decay —ongoing damage to the objects
the player constructed, which he must spend resources to reverse or repair. (Decay
is also sometimes called entropy , although technically entropy refers to increasing
disorder rather than loss of resources . ) Typical decay mechanics look something
like this: “Each section of road includes a numeric attribute indicating its level of
decay as a percentage, with 0 (zero) indicating that the section is new and 100 indi-
cating that the section is fully decayed and impassable. Sections of roads begin to
decay 3 months after they are constructed, and 3 percent is added to their level of
decay every year, plus an additional 1 percent for every 100,000 car trips over the
section in the course of that year. When decay reaches or exceeds 100 percent, the
road section becomes impassable and it must be replaced.”
Because resources are valuable, the player wants to know why a resource disappears
from the world and what benefit compensates for its loss. In Monopoly , players get
money from the bank by passing “Go”—in effect, for no reason at all—but when-
ever a player has to give money back to the bank, the game provides a reason: The
player owes income tax, incurs a fine, or something similar. Players don't mind get-
ting money for free, but when they have to spend it, they want to know why.
Explain your drains.