What is this tool? I found a thin plastic tool, about 5cm long. It is engraved with the number 5. I believe it to be a bike tool. What is it? ![enter image description here](

As linac said: "It's a spacer to be inserted between the brake pads while the wheel is removed"

Andy P has explained why the spacer is needed.

There are a variety of spacers made to work for different disk brake designs.
Here are two examples:

![enter image description here](

Here is a picture of the spacer being used
![enter image description here](