Fill means to make something full by putting things into it until there's no more room. When you fill a glass with water, you pour until the glass can't hold any more. When you fill a backpack with books, you pack it until everything fits inside.

You can fill physical spaces (like filling a bucket with sand at the beach) or you can fill time (like filling an afternoon with activities). Sometimes people talk about feelings filling them up too: joy might fill your heart when you see an old friend, or nervousness might fill you before a big performance.

The phrase fill in means to complete something by adding missing information, like when you fill in the blanks on a worksheet. To fill in for someone means to do their job temporarily while they're away: if your teacher is sick, a substitute teacher fills in for them.

When something satisfies you completely, you might say it was filling, especially about food. A bowl of oatmeal is more filling than a handful of crackers because it makes you feel full longer.

You can also fill a role or position, like when a coach needs to fill an open spot on the team. When doctors fill a prescription, they prepare your medicine. Each use shares the same basic idea: making something complete by adding what belongs there.