1. Why does a bone turn rubbery when the mineral is dissolved?
Scientists who study ancient bones sometimes dissolve the mineral on purpose. To pull the protein collagen out of a bone, a team at the University of York in England first dissolves the rigid mineral parts in a mild acid over several days. After that step, the bone is no longer hard and becomes soft and bouncy like rubber [1].
So the hardness of a bone comes from something that acid can remove. The medical reference MSD Manual explains that the hard outer part of a bone is made mostly of proteins such as collagen plus a substance called hydroxyapatite, which is built mainly from calcium and other minerals. Hydroxyapatite is largely responsible for the strength and density of bone [2]. In other words, the acid takes away the mineral and the stretchy protein stays behind. But if mineral is the hard part, why isn’t a bone just solid mineral?
2. Why does a bone need both hard mineral and stretchy collagen?
Under a microscope, a bone is a composite, a material made of two different ingredients: tiny mineral crystals and collagen molecules [3]. A simple way to picture it is a brick wall held together by ropes. Bricks alone can be hard but crack under a sudden hit; ropes alone would stretch too much. Together they make something hard that does not shatter easily. That picture is just a way to imagine how the parts work together.
Researchers do not yet have the whole story. An engineering lab at Hokkaido University in Japan notes that people have tried to build artificial bone by copying this structure, but it has not worked yet, because the exact mechanical role of collagen, especially how it interacts with the mineral, is complicated, happens at a tiny scale, and is still unclear [3]. So even the question “why is bone hard but not brittle?” is still being studied. Next, how does a bone arrange this material?
3. Why is the inside of a bone full of tiny holes?
If you could slice a bone open, you would see two zones. The hard outer part is called cortical bone. The inner part is called trabecular bone, and it is softer and less dense, like a sponge. Even so, it adds a great deal to the bone’s strength, and if the amount or quality of trabecular bone drops, the risk of breaking the bone goes up [2].
The gaps in that sponge have an important job. Bone marrow, the tissue where blood cells are made, fills the spaces inside the trabecular bone [2]. A bone is not a solid stick. It is a hard shell around a lattice, and the lattice is also a place where the body does its work. Is that lattice a fixed design, though?
4. Do bones keep changing, and what happens when they carry less weight?
They do. The MSD Manual describes bone as a constantly changing tissue whose structure adjusts throughout life in response to the loads it carries [2]. The US Surgeon General’s report on bone health calls the replacing of old bone with new bone at the same spot remodeling. It goes on throughout life, and most of the adult skeleton is replaced about every 10 years [4]. That figure is an overall average for adults, so some bones are renewed faster and others more slowly.
Space travel shows what happens when bones carry less weight. In a NASA research summary about astronauts on long missions, bone density in regions such as the lower spine and the hip fell by about 1 to 2 percent per month on average. The loss varied a lot between people and between bones, and it happened even though the crew did regular exercise in flight [5]. The report links this to remodeling getting out of balance, with a lot of bone being broken down and little new bone being built [5].
Put together, bone hardness comes from three things working at once: a mineral and collagen mix, a hard shell around a lattice, and constant renewal that depends on use. No one of them explains it alone.
5. How can you safely compare a vinegar bone with a water bone at home?
Do this with an adult. Remove any meat from a cooked chicken wing bone, wash it well with soap and water, and set up two jars with lids. Put white vinegar in one and plain water in the other, add one bone to each, and close the lids. Leave the jars for several days. Every day or two, ask an adult to take the bones out so you can compare them by pressing gently, then wash your hands. How many days it takes depends on the bone and the vinegar, so keep your own notes rather than expecting a set number.
The water jar is your comparison: it shows what soaking alone does, so any difference you see comes from the acid. The bone may snap if you bend it hard, so be gentle. Wash your hands after touching either bone, keep vinegar out of your eyes, and never eat the bones or the liquid. Ask an adult to throw everything away when you are done. A bent bone in a jar is not the same as a living bone, which also has blood, cells, and constant remodeling. To go deeper, the sources below include a university lab page on bone as a composite material and a NASA summary on bone loss in space.