You probably know that density is equal to an object’s mass divided by its volume. So if two objects weigh the same amount, the smaller one is more dense.

If you want to make a hundred-pound weight out of Styrofoam, you would need a HUGE volume of material (about a cubic meter, or the size of a small refrigerator). But if you want to make a hundred-pound weight out of solid gold (a very dense metal), it would be just a little smaller than a gallon of milk.

For foil, it’s a bit more complicated. Foil is made of aluminum. Aluminum is not as dense as gold, but it is still pretty dense (1 cubic centimeter of solid aluminum weighs 2.7 grams, so its density is 2.7 grams per cubic centimeter). But foil isn’t a 3D block of aluminum. On its own, foil is very thin, and it is usually folded and scrunched up to form 3D shapes. When that is the case, the volume of the foil structure doesn’t contain only aluminum- it also has big pockets of air. These air pockets add volume, but they don’t add any mass to the foil. Since density is mass divided by volume, the air pockets make the density smaller.

If you compress the ball of foil, you will get rid of some of the gaps filled with air. The volume will get smaller, the mass won’t change, so you will increase its density. If you could squeeze it really, really hard (with way more force than a human could possibly exert), you would end up with a tiny ball of pure aluminum, with no air gaps, and its density would be back to 2.7 grams per cubic centimeter.