Just before saying something important, you take an arrow to the mouth which seals your lips together, making it so you can't talk. Someone doesn't want you to share what you know with someone else.
To remember the reading ち we use the word "cheese."
One of the things you know is actually very strange but very important. The president's cheese (ち) has been poisoned and only you know. If you could just say something then maybe you could save him... but... you have an arrow in your mouth.