We’ve been letting Bear watch Sid the Science Kid in the evenings. The timing of the show isn’t great.  I wish it were on in the afternoon, but I don’t have a way to record it.  One week, every episode dealt with the human body.  The day of the digestive system, Sid and his friends did a fast experiment to show how your stomach digests food. 
You need 1 lemon, a ziploc bag, and a cracker, a knife, and a hand held juicer or just strong hands for squeezing the lemon.
First, slice the lemon in half.
(I was supervising.  She is VERY good at cutting and I really don’t worry too much anymore, though my husband isn’t used to seeing her with sharp knives and flinches as she cuts.)
Put the juice in the ziploc bag.
Break the cracker up a bit (to simulate the chewing) and put it in the bag. 
Squeeze the bag (I mean stomach) for a while and watch what happens to the cracker.
Then record what you saw. 
I was quite impressed that Bear was able to draw so much of it.  I was busy cleaning up and missed the whole process.  But she dictated what I should write after she’d drawn.
If you click on the picture it may come up bigger.