How do you convincingly tell your clients that you don't know the task without them losing faith in you?

[Btaliat]

Sometimes, freelancers may find it very difficult to say the truth. They don't know how the clients will feel. Even when they don't know how to handle the tasks, they may not want to tell the clients. They may be scared that such a client may not give them task in the future again. And the fact is that theybmay need to say the truth at least to be honest with their clients. What do you say about this?