To add to Plato's reply, we can take , which is less than , which is less than...
... , which is less than... etc.
Any fraction of the form , where n is a whole number, will get closer and closer to "1.000" as n gets larger.
But you might want to leave that part out of the discussion with the 3rd grader...
Yes, but you'll run out (at least if you stick to the pattern n/(n +1)) after 1/2. Or maybe 0/1!
What you can do is to increase the denominator (bottom) while leaving the numerator (top) the same.
This would make a bunch of sense to a 3rd grader if you expressed the number of pizza each person got at a party with a fraction...
1/7 means 1 pizza for 7 people.
2/5 means 2 pizzas for 5 people. If we only have 2 pizzas but keep inviting more and more people, we get...
2/20 --> 2 pizzas for 20 people.
2/100 --> 2 pizzas for 100 people...
etc.
Clearly you can tell that each person will only get a bite after increasing the denominator sufficiently!