Hi Jittzy! This is a known issue with some mobile users when using any HTML game & text boxes - most common with android users.
Some users have been able to correctly pass the riddle by copy-pasting the answer. Please ensure you do not have spaces before or after.
Copy paste: silence
OR
Copy paste: Silence
There is a built in mechanic if the game notices you've died on that riddle a few times you will get the option to skip if you do not want to copy-paste.
Sorry!