20th Century Frog

In 20th Century Frog, the player controls a frog whose goal is to eat a number of prey animals. These are usually flies, but bees, locusts and bats also appear. In most levels, the eating is done by jumping in such a way that the frog passes by an insect - the frog's tongue will then automatically snatch it. The level finishes when all animals are eaten. Eating raises the score, while jumping lowers it. A large negative score can actually kill the frog, so fruitless jumps should be avoided. Things are complicated further by deadly hazards unique to each level, including wooden barriers, a stomping foot, a roller, a hungry stork, a spider's web, and many more.

Besides the prey animals, there are other creatures that can be eaten, either for points or for additional effects. A dragonfly can move the frog a level ahead or a level back, depending on its color. Eating a ladybug gains the frog a so-called gimmick. Gimmicks are items that are collected in a sub-menu and can be used at the right point to launch an effect. Many can be used to counter a specific hazard: a steel helmet protects against the stork, a broom can remove the spider's web, an axe can destroy the wooden barrier, et cetera. Others make it easier to catch prey: a dead body attracts flies, a glass of honey does the same for bees and a steppe fire can startle locusts so they are easier to catch. Attractants can be used to summon specific creatures like caterpillars or ladybugs.

The frog's lives are limited but their number can be increased through mating. Another frog can be summoned through an attractant gimmick or by eating a number of caterpillars. If a caterpillar of the wrong color is eaten, the summoned frog will be of the same sex as the player's frog, making mating impossible. A way around that is the hormone gimmick, which allows a sex change.

The game uses almost all keys of the keyboard to control the frog's jumps. The row of the key determines the height of the jump (the number keys are highest, the bottom letter row the lowest), while the horizontal position of the keys control the direction: center keys make the frog jump straight up, while moving to the keyboard edges cause jumps to the left or right. An exception to this control scheme comes in special levels that take place underwater or in space - in these, the frog is controlled directly through the cursor keys.