Jack and Jill went up a hill
And danced around with laughter,
Jack rolled down and lay on the ground,
And Jill came tumbling after.

"Hey, Jack," said Jill, "I'm feeling quite ill,
And my temperature's now getting hotter;
But I have these new caspules, so if it's not a hassle,
I need exactly five units of water."

Jack now was drained, and it would be such a pain,
To get water he'd have to go higher;
But he bent to her will, going back up the hill,
To get water, although he was tired.

On reaching the well, he suddenly fell,
And shattered his pail on the road,
But spying two lasses with cylindrical glasses,
He saw how to carry his load.

They're negligibly thick, he saw rather quick,
Praising himself for sagacity,
And what made it right was that both had same height,
Though obviously different capacity.

Sixteen and four were their volumes to pour,
And Jack filled the first up with water,
The second had none, but purely for fun,
He still took to Jill what he'd got her.

"Just tell me", whined Jill, as she looked for her pills,
"Why you brought me back sixteen and four?
Five from sixteen, is not easily seen."
But Jack said he'd something to show her.

How can sixteen units be reduced to five using these two glasses?