i dont know what happen to my unity. before, the game object must be click to trigger the effect but for some reason, it trigger whenever I click anywhere? what happen? pls help… 
same thing happens even if I created a new project…
No problem here, that’s the expected behavior.
Input.GetMouseButomDown will return true if the mouse is pressed, no matter where. Check the documentation.
If you want that it triggers only when it’s over your object, you have to check it with a ray cast.