Exciting gadget to expand your spy collection with the invisible ink pen

This invisible ink pen gives you the opportunity to write secret messages. No one will know it! If someone wants to send a letter but you do not want every to know what you have written, use the invisible ink pen. Write the letter as usual. You will see that the letters disappear. The disappearance process depends on the speed you write and the temperature of the environment. The person who receives it must keep the letter in blue light and the characters will reappear. People who receive the letter in hand will think that it is a blank paper!