Soda Constructor has nothing to do with soda (as in fizzy liquid), but it certainly evokes a refreshing "wohba!"
It's kind of a 2D physics/robotics gallery and lab. You can choose to browse through the hundreds of creations in the zoo, or take the plunge and build something yourself. I recommend starting here and choosing "click here to play". Sliders on the left control the time, gravity, friction, and kinetic energy - just mess with them, you'll see what they do.
I like to browse through the zoo, pick things up and drop 'em, or turn them over and watch 'em squirm.