Central Washington University. I am modifying it slightly to make it a little more active. No equipment is needed.
Players gather, not necessarily in a circle. However many players there are, the countdown begins with that number. So if there are 10 players, start the countdown at 10. Someone is designated the Mission Commander. The Mission Commander says 'Countdown' and someone begins with the highest number. One at a time, players say each lower number until the group gets to zero. Important: they are not going around the circle in order. The randomness of who is calling out the numbers is the point of the exercise. If two or more players say the same number at the same time, the Mission Commander cancels the launch and begins again. If there is too long of a delay between numbers, ditto. Make sure everyone is participating.
My tweak is to have the players jump up or otherwise do some sort of activity when they call out their number to add some physicality to the warm-up.