Video narration
Then there is the clock. Theta measures what happens to the option's value as time passes. Even if the stock barely moves, waiting can cost a long option value; that is why theta is usually negative for a typical long option. For a short position, or a different contract, the sign can differ. For a short-term estimate, put the three effects together: ΔV ≈ Δ·ΔS + 1⁄2Γ(ΔS)2 + ΘΔt. That means option-value change is approximately delta times the stock move, plus gamma's adjustment for the size of that move, plus theta times elapsed time. It is a local approximation, not a complete pricing model. The Greeks themselves change with price, time, volatility, and other inputs. Here's the flip: calling the direction correctly is not enough. You also need to know your speed, how that speed is changing, and what the clock is charging. So, why can the right directional bet still lose? Because delta captures the move, gamma changes the exposure, and theta can drain value while you wait.