A sleepover, also known as a pajama party or a slumber party, is a party most commonly held by children or teenagers, where a guest or guests are invited to stay overnight at the home of a friend, sometimes to celebrate birthdays or other special events. A lock-in is a similar event held in a setting other than a private home, such as a school or church.

Typical participant activities include staying up late, talking, eating and playing until falling asleep. Sometimes sleepovers involve another activity prior to the actual sleepover. For example, dinner at a restaurant, skating, or other scheduled events. Sleepovers are usually held at one participant's house, with other guests sometimes bringing their bedtime things, such as pillows or sleeping bags. Common activities include playing board games or video games, having pillow fights, watching movies, midnight feasts, playing party games such as Truth or Dare?, light as a feather, stiff as a board, and spin the bottle, building forts out of pillows and blankets, or having a "spa night", in which participants polish their nails and toes and put on facial masks. Apart from bedrooms, sleepovers can be conducted in tents, trailers, and on trampolines.

A party is a gathering of people who have been invited by a host for the purposes of socializing, conversation, or recreation. A party will typically feature food and beverages, and often music and dancing as well.

Some parties are held in honor of a specific person, day, or event (e.g., a birthday party, a Super Bowl party, or a St. Patrick’s Day party). Parties of this kind are often called celebrations.

A party is not necessarily a private occasion. Public parties are sometimes held in restaurants, pubs or bars, and people attending such parties may be charged an admission fee by the host.

A birthday party is a celebration of the anniversary of the birth of the person who is being honored.

Birthday parties are a feature of many cultures. The guests may be asked to bring a gift for the honored person. In Western cultures, birthday parties are often accompanied by colorful decorations, such as balloons and streamers. In these cultures, a birthday cake is usually served with lit candles that are to be blown out after a "birthday wish" has been made. The person being honored will be given the first piece of cake. While the birthday cake is being brought to the table, the song "Happy Birthday to You" or some other dedicated birthday song is sung by the guests.

A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine. Geared devices can change the speed, torque, and direction of a power source. The most common situation is for a gear to mesh with another gear, however a gear can also mesh a non-rotating toothed part, called a rack, thereby producing translation instead of rotation.

The gears in a transmission are analogous to the wheels in a pulley. An advantage of gears is that the teeth of a gear prevent slipping.

When two gears of unequal number of teeth are combined a mechanical advantage is produced, with both the rotational speeds and the torques of the two gears differing in a simple relationship.

In transmissions which offer multiple gear ratios, such as bicycles and cars, the term gear, as in first gear, refers to a gear ratio rather than an actual physical gear. The term is used to describe similar devices even when gear ratio is continuous rather than discrete, or when the device does not actually contain any gears, as in a continuously variable transmission.