The vastness of the universe is so staggering that we can’t even imagine what infinity is. And yet we get intimation of the universe in a grain of sand and a glimpse of eternity in a moment.
Consider light. The smallest quantum of light is a photon. Photons are what our senses perceive to see light. In its default state, light is white which when refracted differently gives us the entire spectrum of colors. Red is on the higher end of the visible spectrum and violet the lower end. There are colors which our sense do not perceive, like ultra red and ultra violet rays. Yet, of the colors visible to the human eye, it can distinguish 10 million different shades.
Or, consider sound. Sound is made up of phonons. Each sound we hear is actually made up of several phonons so that what we perceive is a complex of many sounds. I have been experiencing failing hearing. And of late, I notice that much of the music I hear sound out of tune. It must be because of my hearing loss, I do not hear the higher pitched sounds and therefore do not hear anymore the melody correctly.
Or, consider what is generally accepted at the origin of the universe. Everything that exists, both what is visible and invisible to us, originated from that densely packed singularity that exploded at the Big Bang. One can imagine that at least one of the particles in my body was part of that big bang; because matter, once brought into existence, never dies nor gets destructed. It may change shape or form but matter persists.
What is space but for matter to move around in. What is time but a measure of what it takes matter to get from here to there. And it again one can imagine, there was once at time everything was but one singularity in one place and in one moment; then indeed it is possible to see the universe in a grain of sand and eternity in a moment.