Here's a general transcript of a school assembly I just gave to 5th and 6th graders in Newbury Park, California:

We are here today to talk about Music which is made up of melody, harmony and rhythm.

Rhythm is a repeated pattern or movement. [definitions differ yet this is the shared notion]
This can be found in our walking or in our heartbeat.
In order to feel your heartbeat, put your wrists up to your face and cross one hand in front of the other as if it were a wrist-watch. Let your fingers curl around your wrist so that the tips of your fingers are right below where you thumb meets your hand.
The beating of your heart is a constant pulse that changes pace depending on how much activity you are doing. If you are resting, the common pulse is around 60 beats per minute to 80 beats per minute. 60 beats per minute is also known as 1 beat every second.
This corresponds to the rhythm of our Earth spinning--and therefore the rising and setting of the sun--in that our units of time measurement are in seconds, minutes, hours, days and years. This is the rhythm of nature of which we are directly in tune!

Rhythm is like a language in that there are basic units of rhythm (words), which make up phrases, followed by sentences, which are arranged into stories.
The simplest form of rhythm is characterized by the number 1. [Students instructed to clap a regular pulse of 1 or quarter notes]
Then, when you double that 1, you get groupings of 2. [Clap double the speed of 1s - the teacher is aware that these are 8th notes but the students are just clapping along counting 1 2 1 2 1 2 etc.]
The next is the number 3. [Students follow along with clapping triplets saying 123 123 123 etc.]

All these groupings can be played over a pulse of 1. [Play pulse slowly in bass drum and instruct students to clap 1s, followed by 2s (over the teacher's pulse of 1), and finally by 3s over the pulse of 1. You may have to stop in between tries to get everyone to clap together.]

These three basic units of rhythm--1, 2 & 3--make up every more complicated rhythms.
For example, groupings of 4s [16th notes] are just doubled 2s.
[Bring up to students the fact that they are learning math. Ask how many times 2 goes into 6] Groupings 6 have both 2 sets of 3s or 3 sets of 2s.
If you take an odd grouping like 5, you will have a remainder: there are 2 groups of 2s plus 1 left over which makes a group of 2 and a group of 3. [Play pulse of one and demonstrate 5-tuplets. Same with 7-tuplets; they are grouped into 2 - 2 - 3 or 3 - 2 - 2 or 2 - 3 - 2.]
[Ask: how many times does 2 go into 8? They will respond: 4] Demonstrate 8 notes over the single pulse and then double that to get 16. When played at a quarter note pulse of under 60 beats per minute, they will love it when you play 64th notes, most easily played by playing a double stroke roll over 32nd notes, or whatever ;)

Improvise groupings over pulse. You can play simple groupings while they clap the quarter note pulse with your bass drum and then get busier, always keeping the bass drum pulse going so they feel like they are playing along with you!

STYLES or MUSICAL APPLICATION: Rhythm is a part of all music, but as a drummer, your job is to accompany the molody and harmony so everyone feels comfortable. Depending on what style of music you are playing, a different orchestration of rhythm will be necessary.
In Cuban music, the drum set player is one player that brings together the rhythms originally played by multiple players. The cascara is played on a wood shell, the tumbao rhythm is played on a bombo or big drum and the clave rhythm (played on 2 wood sticks) can be played over a rim click, or hihat foot.
If I were playing in a modern American/Jazz Afro-Cuban feel, I might end up playing a straight 8th jazz ride pattern with the clave in the hihat foot, the tumbao in the bass drum and the cascara in the rim click [if I were to play extremely busily].
Play Cha-cha and various other styles.
Afro-Cuban music can have a pulse of duples or triplets, and you can switch between them. [Demonstrate 6/8 feel to 4/4 guaracha feels and how to go in between them.]

[Teacher asks to play African Rhythms] Since Africa is the "seat of civilization" for many reasons, rhythms of Africa can be found in every style of music. In playing the Cuban rhythms, I was already playing rhythms that are inherent in the music of Africa. [addendum: common rhythms are found in all styles of music, simply because we all have hearts beating and the spinning of the Earth as our shared experience.]
One way rhythms were used in Africa was to communicate over vast distances.
I follow this by playing 6/8 African bell pattern followed by dotted quarter 4-feel on tom, followed by playing call and response solo patterns between toms and snare (snares on or off) over pulse.

Continue with asking students for styles of music that they would like to hear. You will inevitably play some of the same beats but you will be able to tell stories and give knowledge of the specifics of each style. For example, hip hop and rap will come up and you can relate the knowledge that these musics began by incorporating samples which are beats from popular grooves from styles like soul, rock, funk, etc. [Play "Funky Drummer" break by James Brown to exhibit most sampled groove in history. Rock and metal are close relatives; demonstrate variations of common grooves.

In this discertation, the commonality of rhythms throughout the world's styles will be apparent, especially after beginning the assembly with the clapping of pulses 1, 2, and 3s over the common, singular pulse.