Q.A coin is tossed twice. Write the sample space of this random experiment.

A random experiment is an action with more than one possible outcome where the result cannot be predicted in advance, even though every possible outcome is known beforehand — tossing a coin or rolling a die are the standard examples, as opposed to a deterministic experiment (e.g. heating water to $100^\circ\text{C}$), which always gives the same result. A single performance of a random experiment is a trial, and its result is an outcome. The sample space $S$ is the set of all possible outcomes of a random experiment, and each individual outcome is a sample point. $S$ may be written in roster form, e.g. $S = \{H, T\}$ for one coin, or in set-builder form, e.g. $S = \{(x,y) : x, y \in \{1,\dots,6\}\}$ for two dice thrown together, which has $n(S) = 36$ ordered-pair outcomes, since the two dice are treated as distinguishable.