Steins;Gate - Season 1 Episode 6 -

The lab members discuss how to perfect the Mobile Microwave, dubbing messages sent to the past as 'D-Mails'. Further experimenting reveals the various rules concerning D-Mails; being that it can only be sent during a certain time of the day, long messages are split into several smaller mails, and the timer on the microwave determines how far back the message is sent. The next day, Moeka comes round to see the IBN 5100 and, after hearing about the time experiments, is made a lab member.