A kid-targeted sci-fi/adventure movie, released in 1985. I grew up with this movie, so it holds a special place in my heart, right up there with Flight of the Navigator, The Goonies, and The Neverending Story. Explorers stars Ethan Hawke, River Phoenix, and Jason Presson as three young kids; Hawke plays Ben, the dreamer, Phoenix plays Wolfgang, the scientist, and Presson plays Darren, the down-to-earth one. Superb acting from all three of them, Phoenix especially.
The storyline goes like so: Ben starts having a series of inexplicable dreams about some sort of computer circuit, which his buddy Wolfgang somehow throws together in his lab (incidentally, the technical accuracy of the whole movie is a little questionable, but I'm willing to overlook it). Turns out this circuit creates a "point of force", or in practical terms, an invisible sphere which can move around and accelerate without feeling the effects of inertia.
Sooner or later, they and their friend Darren figure out they could potentially tool around in this thing, and whip themselves up a homemade spaceship out of an old Tilt-o-Whirl and some other scrap from the junkyard, and go cruising around town. Eventually, however, the ship's computer goes haywire and they're warped a huge distance through the cosmos, confronting another ship about a thousand times the size of their own.
Unfortunately, the rest of the movie isn't even worth describing, as it goes really downhill from there--a whole bunch of really lame interaction with some totally unbelievable aliens. If you end up watching this movie (and I recommend you do, if you like these sorts of movies), just stop watching when they reach the big spaceship. Seriously. I'm not even kidding. Turn it off there.