What is the main secret to writing a subject line for a cold email that makes people curious and want to open it right away?

The main secret to writing a subject line for a cold email that makes people curious and want to open it right away is to immediately establish direct, specific personal relevance to the recipient, coupled with a concise hint of potential value or insight, without fully revealing the email's content. A cold email is an unsolicited message sent to a recipient with whom the sender has no prior relationship, and the subject line is the brief text displayed in the inbox indicating the email's topic. Personal relevance means the subject line clearly signals that the email is specifically for *themand relates to their known interests, role, company, or challenges, often discovered through prior research into the recipient's public profile or business activities. Curiosity is the desire to know more, and it is triggered when the subject line presents an informational gap by suggesting there's a specific, relevant solution or valuable insight waiting inside the email that pertains directly to the recipient. For instance, instead of a generic subject like "Quick question," a relevant and curiosity-inducing subject might be "Idea for [Recipient's Company] regarding [Specific Challenge They Face]" or "Question about your recent [Project/Article/Tweet]." This approach is effective because it avoids generic pitches, which signal irrelevance and often lead to being ignored, and instead positions the email as a direct, individualized communication. The partial disclosure of specific value or insight compels an immediate open to resolve that informational gap and potentially gain a benefit directly relevant to their situation.