Milk contains lactose, two simple sugars (glucose and galactose) glued together. Our bodies make the enzyme lactase, which breaks lactose into its 2 parts. Enzymes are little molecular machines that do stuff in our bodies. Our bodies can easy use simple sugars like glucose as energy sources.
If people don't have enough lactase, milk can upset their stomachs. Modern medicine helps: Lactase tablets provide the enzyme for you. Lactaid milk comes with all the lactose already broken down.
I did an experiment with glucose test strips, crushed lactose tablets, milk, water, and cream.
I poured out pure milk (no lactase), milk with lactase, water with lactase, and cream with lactase.
I covered the "test tube" containers with aluminum foil and held the containers in warm water for a few minutes (enough time to change out of pajamas and into regular clothes)
Then, after changing clothes, I uncovered it an tested them with the glucose test strips. These are the results after a few seconds:
Then I left the house and came back later.
Blue means no glucose. Green means a little bit of glucose. The darker the brown, the more glucose it has.