Nano-recorder

Nanorecorders were personal audio-recording devices used by the Eleventh Doctor. They were small devices implanted in the palms of the users' hands.

Use
In preparation for fighting the Silance, a race of aliens with the power to erase themselves from the memories of all who encountered them, the Doctor injected nanorecorders into the palms of Amy Pond, Rory Williams, River Song, Canton Delaware, and himself with a pistol-like injector.

When activated by the user pressing on their palms, the devices recorded all the user's speech via a telepathic link to the speech centres of the brain. The nanorecorder then glowed with a pulsing red light to indicate it had been used. In this way, when one of the TARDIS crew encountered the Silence, they could describe the encounter aloud and record it to their nanorecorder. Afterward, when they had forgotten the encounter, they would be alerted to it by their glowing palm.

When the Silence abducted Amy, they removed her nanorecorder and left it behind. Rory found the nanorecorder, which had defaulted to live mode, allowing him to hear Amy's speech live via the nanorecorder's telepathic link.

After the Silence were defeated, Amy caught Rory using the nanorecorder to eavesdrop on her conversation with the Doctor and chided him for it. (DW: Day of the Moon)