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Shade 3/8 of the bar.

3 of 8 equal parts.

Fill the blank so the LED blinks.

1from gpiozero import LED
2from time import sleep
3
4led = LED(17)
5while True:
6led.()
7sleep(0.5)
on()toggle()stop()

Tap the chamber that pumps blood to the whole body.

RARVLALV

Oxygen-poorOxygen-rich

Left ventricle — the thickest wall.

Drag the slider to change the frequency.

y = sin(12x)

1−1π2π3π4π
f

The robot learns the way to its charger.

episode 214reward +1

A collapsing core weighs 3 Suns. What does it become?

A white dwarfA neutron starA black hole

Balance the equation.

2H2 + O2 → 2H2O

Turn any topic into a Duolingo-style course

Visuals and diagrams

The Life Cycle of a Star
A cycle diagram: a nebula forms a star, the star ends in a supernova, and the scattered gas feeds a new nebula.
The Ship of Theseus
Three ships in a row: the original, one with half its planks replaced, and one with every plank replaced.
Neutron Star vs Black Hole
A neutron star firing two beams beside a black hole circled by a ring of gas.
When a Core Collapses
A layered star core splitting two ways: a lighter core becomes a neutron star, a heavier core becomes a black hole.

Practice that sticks

Calculus

What does a derivative measure?

Tap an answer to try it.

Personal finance

Same rate, 30 years. Which grows your savings more?

Tap an answer to try it.

Black holes

Past which boundary can nothing escape a black hole?

Tap an answer to try it.

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What do you want to learn?

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Gravity Pulls Matter Together

A 25-Sun-mass star seems to hold itself together—so what invisible influence keeps pulling all its matter inward?

Frequently asked questions