A German firm is proposing to transmit adverts via train windows so that the sound appears to "come from inside the user's head" when passengers lean against them.
The idea would use bone conduction technology, a technique that transmits sound to the inner ear by passing vibrations through the skull.
"Tired commuters often rest their heads against windows," says the ad.
"Suddenly a voice inside their head is talking to them. No-one else can hear this message."
Oh yes, I can see that really helping a tired commuter. Why wouldn't you want to think you're hallucinating, at the end of a long day?