Ah, childhood. Treehouses and sleepovers and talking to your friends on tin can telephones. It's made of junk: just two old tin cans and a length of string, but it's the stuff that memories are made of. Occasionally you can see a variant that has a tube that travels underground. This variant is derived from early nautical vessels. Truth in Television, of course, although the real thing only works if there is no slack in the string at all. In visual media, the string is just as likely to be portrayed as slack or winding its way around, under, or over things, which wouldn't work.