Rubbing sandpaper reduces/increases friction on a wooden surface.

Explanation

### Step-by-Step Solution:

1. **Understanding Friction**: Friction is the resistance that one surface or object encounters when moving over another. It depends on the roughness and irregularities of the surfaces in contact.

2. **Identifying the Surfaces**: In this scenario, we have a wooden surface and a sandpaper surface. Sandpaper is known for its rough texture, which means it has many irregularities.

3. **Rubbing the Surfaces Together**: When sandpaper is rubbed against the wooden surface, the two surfaces interact. The roughness of the sandpaper and the wood will cause them to wear against each other.

...