Need to flex your mind palace between instalments of hit BBC1 detective drama 'Sherlock'? The game is on!
Sherlock is 'hacking' BBC1's official Twitter account this evening to test your powers of deduction.
'Sherlock' writers Steven Moffat and Mark Gatiss will be tweeting live and you can demonstrate your crime-solving skills by replying using the hashtag.
All you've got to do is solve the case before Sherlock gets bored again and reveals the answer...
'Sherlock' series four concludes on Sunday at 9pm on BBC1.