This is a nearly foolproof (nothing is ever completely foolproof in my experience!) for teaching children to identify whether a piece of music has four beats in a bar, or three beats in a bar. I was amazed at how quickly the children got the hang of the idea when a colleague of mine used it with a year 4 class.
The children need to be standing up. Play the music, and pick up the pulse by stepping on one foot for beat 1, the other foot for beat 2, and clapping on beats 3 and 4, e.g. “stamp stamp clap clap”. It will be easy to count the beats as you do these actions, and it becomes obvious that the music has four beats in a bar. If I haven’t used the phrase “beats in a bar” before, I just say “it counts in 4”. Try this with a number of pieces of music which are all in four-time.
Now, change to another piece, in three-time. Choose something with a fairly steady tempo, or the children won’t be able to keep up! Now, the actions become “stamp stamp clap stamp stamp clap”, and the children should have no problem in recognising that the music now “counts in 3”.
What I thought was so clever about these actions, is that three-time is marked with the same (and dominant) actions on beats 1 and 2; I usually think of three-time as “Om-pah-pah, om-pah-pah”, but by treating it as “om-om-pah om-om-pah”, the switch between four and three time is instantly recognisable.
This is a neat way of preparing instrumental students for the aural test component of grade exams when “counting the beat” is required.