にExistence location marker
に marks the place where a person, animal, or thing exists. In this use, it normally appears with ある or いる.
What the particle marks
Put に after the place itself. The rest of the sentence tells you what exists there or where a known person or thing is.
Things and living beings
Use ある for an inanimate thing and いる for a person or animal.
Saying where someone or something is
Asking where something is
The answer omits the scissors because both speakers already know what they are discussing.
Existence location versus event location
Use に for where a person or thing exists. Use で for where an action or event takes place. The presence of ある alone does not automatically require the existence-location marker.
公園で祭りがあります。
There is a festival in the park.
AnalyzeA festival is an event taking place in the park, so its event location takes で.
More examples
冷蔵庫に牛乳があります。
There is milk in the refrigerator.
Analyze犬は玄関の前にいます。
The dog is in front of the entrance.
Analyze辞書は本棚にあります。
The dictionary is on the bookshelf.
Analyze先生は職員室にいます。
The teacher is in the staff room.
Analyze湖に小さな島があります。
There is a small island in the lake.
Analyze妹は今、台所にいます。
My younger sister is in the kitchen now.
Analyze