Jack has several meanings kids encounter regularly:

1. A tool for lifting heavy objects, especially cars. When someone gets a flat tire, they use a car jack to lift the vehicle off the ground so they can change the tire. The jack slides under the car's frame, and when you pump the handle or turn a crank, it slowly raises the entire weight of the car. Mechanics use larger jacks called hydraulic jacks that can lift even heavier vehicles.
2. A playing card that ranks between the ten and the queen. Each deck has four jacks: one for each suit (hearts, diamonds, clubs, and spades). In many card games, the jack has special value or power. The cards often show a picture of a young prince or royal servant, which is why jacks are sometimes called knaves in older books.
3. A small six-pointed metal piece used in the game of jacks. Players bounce a small ball and try to pick up jacks before catching the ball again. The game requires quick reflexes and coordination.
4. A socket or plug for connecting electronic equipment, like a headphone jack where you plug in headphones, or a phone jack for older telephone connections.

The word also appears in many common phrases: a jack of all trades means someone who can do many different things reasonably well, and to jack up means to raise something suddenly, like prices.