Collective nouns

When many coral colonies grow close together over a very long time, they build up to form a reef. Reefs can get so high they come out of the water at low tide, and can be dangerous to ships.

A group of otters is called a romp.

A group of squirrels is called a scurry or a squad.

A sheaf is a bundle of a freshly harvested cereal crop like rice or wheat. When harvesting was done by hand, the harvest was bundled up into sheafs that were just big enough for one person to carry.

A skein is a name for a flock of geese or ducks when they are flying in a line or V.

A group of bears is called a sloth.

A lot of things put on top of one another in a relatively neat way is called a stack.
When you put things on top of each other in a messy way, it is a pile.

A group of tigers is called a streak, or an ambush.

A team is a group of people or animals working together to achieve a common goal. When sports are played, they are often played in teams.
Members of a team often wear the same uniform so you can see which team they belong to.

A tribe is a group of people with something in common, for example a group of people living in a village with the same leader.