Although eavesdropping is typically frowned upon, if you're just playing around with your friends, siblings or parents--i.e. trying to find out what they got you for your birthday or who your little sister is talking to on the phone--eavesdropping is harmless fun. If your walls are thick and you can't quite get the gist of the conversation, you can try a few other tactics. To listen in on another person's conversation, you can use conventional methods or you can try some high-tech gadgets.
Press a drinking glass onto the wall shared between you and the person you're eavesdropping on. The mouth, or opening, of the glass should be placed on the wall, with your ear press against the bottom of the glass. This will help to improve the transmission of sound from the other room, letting you in on the conversation.
Go to a room above the room that the person is in. Place your ear to a vent or cold air return in the room. The venting system will carry the sound up to you from the room below.
Enlist the help of a few friends. Position your friends in various rooms surrounding the room where the conversation is in. Although you may not be able to hear the entire conversation perfectly, each friend may pick up little bits and pieces of it. Together, you can put the puzzle pieces together and figure out what the conversation is about.
Purchase a sound amplifier. These little gadgets can be purchased on the Internet, late night during infomercials or from a local gadget and/or electronic store. Although these devices are marketed to help those hard of hearing understand the conversations around them, they can certainly amplify even quiet conversations from a few rooms away.
Utilize the technology you already have. Borrow a family member's or friend's cellphone. From your cellphone, call the second phone and answer it yourself. Then, plant the second phone in the room where the conversation is going on. You can then move to a different room and listen-in on your phone.