Can you make foam with just milk?

Yes, you can make foam with just milk by frothing or steaming milk, creating milk foam used for lattes and cappuccinos, which is foam made from milk proteins trapping air bubbles, but you cannot make solid foam like polyurethane foam with just milk. Milk foam is made by agitating milk with whisk, frother, French press, or steam wand, which incorporates air into milk proteins creating stable foam on top of coffee drinks. This works because milk contains proteins and fats that stabilize bubbles. For solid foam, you need chemical reactions that create gas bubbles in solidifying material, which milk alone cannot do. Some experimental bio-foams use milk proteins like casein mixed with other ingredients to create biodegradable foam, but pure milk alone will not create solid foam, only liquid foam that collapses quickly. So you can make edible milk foam for drinks with just milk and air agitation, but not construction or cushion foam, which requires polymers and blowing agents to create solid foam structure.