When it comes to sex, animals have developed a plethora of tricks to avoid mixups with other species. They use sight, smell, and even feel to tell each other apart. But in certain cases, the similarities between species are just too strong, so internal processes, such as a female’s ability to detect sperm from another species and select those that belong to her own, have to pick up the slack. This is what tends to happen with Caenorhabditis worms, a type of roundworm. Unfortunately, this doesn’t always work — sometimes the sperm cells are just too quick, and things can go terribly wrong.