GUILDENSTERN takes a coin out of his bag, spins it and lets it fall. ROSENCRANTZ studies it, announces it as "heads" (as it happens) and puts it into his own bag. Then they repeat the process. They have apparently been doing it for some time.
However, the most masterful poets have been able to escape the seeming rigidity of the meter through small variations, such as adding syllables or beginning a line on a stressed, rather than an unstressed, syllable.
Iambic pentameter and fourteen rhyming lines define the sonnet; however, poets have diversified the form over hundreds of years as they adapted to local languages and styles.