Remember 'at', 'on' and 'in' can be used for locations. At is for a single specific place. On is usually connected to a surface. In is more an enclosed space.
Examples with at:
- They are at school.
- Karl is at work.
- I'm sitting at my desk.
Examples with on:
- We are on the bus.
- Lindsay is on her phone.
- I'm sitting on a hard chair.
Examples with in:
- Lindsay is in the bookshop again.
- The socks are in your drawer.
- The parcel is in the post.