How do you enter that hidden door at the old trainer's house at 7 island in Pokemon Fire-Red?

Question

How do you enter that hidden door at the old trainer's house at 7 island in Pokemon Fire-Red?

Answer 1

''The e-Reader (カードeリーダー, Kādo Ī Rīdā?, Card e-Reader) is a device made by Nintendo for its Game Boy Advance portable video game system. It has an LED scanner that reads "e-Reader Cards," paper cards with specially encoded data printed on them.

Depending on the card and associated game, the e-cards are typically used in a key-like function to unlock secret items, levels, or play mini-games when swiped through the reader. See below for a comprehensive list of cards and their functions.

The e-Reader is neither a console nor an accessory, but an add-on device, like the Famicom Disk System or the Sega CD. The e-Reader is one of only three official Nintendo add-ons to be released in North America. The other two are the Super Nintendo Entertainment System's Super Game Boy and the Nintendo GameCube's Game Boy Player.''

That's exactly what you need to go through that door.Don't worry about t though as you are not missing too much.Basically in that room you fight a few battles with strong trainers and that's all.