Planet of the Its
I learned this game from my friend Shane Kramer, while I was working at Scanlon Creek Conservation Area.
Mark out your playing area either using pylons (if playing outside) or by designating boundary lines (if playing in a gym). Designate a pair of players to start as a two-headed "IT". All the rest of the players are single-headed "THATs". When the IT tags a THAT, the THAT joins the IT, which is now a three-headed IT. When the three-headed IT tags another THAT, it splits into two, two-headed ITs. (There's no such thing as a four-headed IT, only two- and three-headed ITs.) Now the two, two-headed ITs continue chasing the THATs.
THATs need to be warned that their planet is a very small one (indicate the boundaries). If any THAT falls off the planet (i.e. goes out of bounds), they automatically get joined onto the nearest two-headed IT.
Play until only two single-headed THATs left. They become the first two-headed IT in the next round of the game.