A quantum program in OpenQASM 3 declares its qubits and its classical bits, applies gates, and measures:

```qasm
OPENQASM 3.0;
include "stdgates.inc";
qubit[1] q;
bit[1] c;
c = measure q;
```

Every qubit starts in the state |0⟩. A gate changes the state; a measurement reads one bit from each qubit it measures. This program applies no gate, so its measurement reads 0 every time.

A qubit's state is two numbers, the *amplitudes* of |0⟩ and |1⟩. Measuring gives 0 or 1 with the squares of their sizes as probabilities. The state |0⟩ has amplitudes 1 and 0, so 0 comes up with probability 1.

**How the check works.** The exercise states what your program must do, here an exact distribution of its measured bits. The engine computes your program's distribution exactly and compares; its verdict says how it checked (by proof, when the distribution is computed exactly) and whether the program does what is asked. Nothing is sampled and nothing is guessed.

On the command line:

```sh runnable
wai-quantum learn start first-circuit first-circuit.qasm   # the starter program, to edit
wai-quantum learn check first-circuit first-circuit.qasm   # exits 1 until the program passes
wai-quantum learn hint first-circuit first-circuit.qasm    # the next hint
wai-quantum learn why first-circuit first-circuit.qasm     # what went wrong, from the engine's verdict
```

In the Studio, the course's **Check**, **Hint** and **What went wrong?** do the same with the program in the editor.