Some of the body's cells stay put for life, while others are free to roam. To move, these migratory cells rely on filopodia—sensitive, finger-like protrusions that reach out from the cell membrane into the local environment. In a healthy cell, this can be a lifesaver: say, when an immune cell is speeding to the site of an infection. But filopodia can also wreak havoc: metastatic cancer cells use them to invade new regions of the body.