Every fact in the three skeleton lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Simplifications and care points: this is general science education, not medical advice. The number 206 is the usual count of named bones in an adult skeleton; the exact number varies a little between people, and children have more separate bone pieces that join as they grow. The forearm calculation uses the textbook's simplified model (forces vertical, forearm horizontal, weights at fixed points). The paper-tube test shows that shape matters for strength; it is not a model of the real material of bone. Pupils with an injury, a cast or a condition that affects their bones or joints can record or observe instead of doing the movement parts; questions about a pupil's own bones go to a parent, the school nurse or a doctor.
Type: National health agency training module
The adult human skeleton usually consists of 206 named bones, grouped into the axial skeleton (80 bones: head, vertebral column, ribs and sternum) and the appendicular skeleton (126 bones: the free limbs and the girdles that attach them to the axial skeleton).
Used in: Ages 10–11 (quiz); Ages 10–11 (text); Ages 15–16 (quiz); Ages 6–7 (quiz); Ages 6–7 (text)
https://www.training.seer.cancer.gov/anatomy/skeletal/divisions/
Type: Peer-reviewed open textbook
The skeleton supports the body (without it the body would be a limp mass), facilitates movement, protects internal organs (the ribs protect the lungs and heart, the cranium the brain), produces blood cells in red marrow, and stores and releases minerals (especially calcium and phosphorus) and fat (in yellow marrow). Bones act as levers and joints as fulcrums; a bone moves only when a muscle spanning the joint contracts. Cartilage gives flexibility and smooth surfaces where bones move.
Used in: Ages 10–11 (quiz); Ages 10–11 (text); Ages 15–16 (experiment); Ages 15–16 (text); Ages 6–7 (experiment); Ages 6–7 (quiz); Ages 6–7 (text)
https://openstax.org/books/anatomy-and-physiology/pages/6-1-the-functions-of-the-skeletal-system
Type: Peer-reviewed open textbook
The 206 bones fall into five shape classes. Long bones (femur, tibia, fibula, humerus, ulna, radius, metacarpals, metatarsals, phalanges) function as levers. Short bones (only the carpals and tarsals) give stability with limited motion. Flat bones (sternum, ribs, scapulae, cranial bones) anchor muscles and protect organs. Irregular bones (vertebrae, many facial bones) protect organs such as the spinal cord. Sesamoid bones sit in tendons; the patellae are the only ones everyone has.
Used in: Ages 10–11 (experiment); Ages 10–11 (quiz); Ages 10–11 (text); Ages 15–16 (text)
https://openstax.org/books/anatomy-and-physiology/pages/6-2-bone-classification
Type: Peer-reviewed open textbook
Compact bone is the denser, stronger tissue and withstands compressive forces; its unit is the osteon, rings of calcified matrix around a central canal with blood vessels and nerves. Spongy bone is a lattice of trabeculae that form along lines of stress and make bone lighter. The hollow medullary cavity of a long bone is filled with yellow marrow; red marrow in some spongy bone makes blood cells. Collagen fibres give bone flexibility so it is not brittle; hydroxyapatite crystals (from calcium phosphate and calcium carbonate) give hardness and strength. Osteoblasts form new bone, osteocytes maintain it, osteoclasts break it down.
Used in: Ages 10–11 (experiment); Ages 10–11 (quiz); Ages 10–11 (text); Ages 15–16 (quiz); Ages 15–16 (text)
https://openstax.org/books/anatomy-and-physiology/pages/6-3-bone-structure
Type: Peer-reviewed open textbook
The early embryo's skeleton is fibrous membranes and hyaline cartilage, which bone later replaces; ossification starts by the sixth or seventh week. Long bones grow in length at the epiphyseal (growth) plate, a layer of cartilage, until early adulthood; then only the epiphyseal line remains. At birth the skull and collarbones are not fully ossified and the skull sutures are not closed.
Used in: Ages 10–11 (quiz); Ages 10–11 (text); Ages 15–16 (quiz); Ages 15–16 (text)
https://openstax.org/books/anatomy-and-physiology/pages/6-4-bone-formation-and-development
Type: National health research agency
Bone is living tissue that changes constantly: old bone is removed and replaced with new bone. In childhood and the teens much more new bone is deposited than removed; bone mass usually peaks by the mid- to late 20s. Calcium (milk, cheese, yogurt; also some fish, leafy green vegetables, grain foods) and vitamin D help build bone; weight-bearing activities such as hopping, skipping, jumping, running, dancing and team sports help too.
Used in: Ages 10–11 (text); Ages 15–16 (text); Ages 6–7 (quiz); Ages 6–7 (text)
https://www.niams.nih.gov/health-topics/kids-and-their-bones
Type: Peer-reviewed open textbook
Example 9.4: with the biceps attached 4.00 cm from the elbow, a 2.50 kg forearm with its centre of gravity at 16.0 cm and a 4.00 kg book at 38.0 cm, balancing torques about the elbow gives a biceps force of 470 N, about 7.38 times the 63.7 N weight supported; the elbow joint force is 407 N. Muscles exert far greater forces than we might think; most skeletal muscles exert much larger forces inside the body than the limbs apply outside, and muscle systems are simple machines with mechanical advantage less than one.
Used in: Ages 15–16 (experiment); Ages 15–16 (quiz); Ages 15–16 (text)
https://openstax.org/books/college-physics-2e/pages/9-6-forces-and-torques-in-muscles-and-joints