This was a super fun activity to do with my girls. The 2 1/2 year old LOVED rolling her gumball down the tubes over, and over, and over... She would hear the "plink" of the gumball into the plastic bowl and run over to see which bowl it landed in.
At times one side was favored, but after some adjusting it was hard to say just what side the gumball would shoot out.
So, first you need to build your tube tunnel. Hopefully you've been saving up those paper towel and toilet paper rolls! Use masking tape to secure the first tube to the back of a chair, the counter, or somewhere elevated.
Tape tubes together until you have formed a tunnel. We made out tunnel with two entrances and two exits for more variation. When you feed two routes together, cut a hole in one tube and snip off the end of the other tube at an angle. Insert the angled tube into the hole. This allows for a pretty clear tube route (so the inserted tube doesn't block the route). Be sure to secure the tube tunnel onto an object for stability in a few places along the tunnel route. Place a bowl under each exit.
Now let your child roll a small rubber ball, gumball or other small round object through the tube. Keep track of how many times the ball falls into the right/green bowl or into the left/red bowl during 10 runs. What number of times did the ball roll into the green bowl? ...and into the red bowl? Multiply that number by 10 and this is the percentage of times.
Probability is the measurement of how likely an event (one or more outcomes of an experiment) is. (http://www.mathgoodies.com/lessons/vol16/intro_probability.html)
The probability of event A is the number of ways event A can occur, divided by the total number of possible outcomes.
P(A)= The Number of Ways an event can occur
.........The Total Number of Possible Outcomes
P(red bowl) = 1 (#ways ball can fall into red bowl) =1/2
This is a bit over a young child's head, but it's helpful to understand a bit of the concept so you can teach PROBABILITY.