Just for a second, let's imagine that your ultimate sexual fantasy is to be doused in custard while standing on a salmon as you hum the Timesteps symphony from A Clockwork Orange. It's left-field, we admit, but we won't judge. The question is, how would you go about telling your partner or partners about such a fantasy? After all, unless you're very confident about their reaction, telling them during a Saturday morning lie-in might cause howls of anger or derision. That's why a German startup has built UnderCovers, an app that is designed to help couples who, for whatever reason, might struggle to communicate their innermost desires to one another.