Appropriate Level: Algebra 2 (with logarithms)
The Question: Suppose you start with an object that has the mass of a proton. If this object doubles in mass every five minutes, how long will take for it to grow to the mass of the current universe?
Ask for guesses. Will it be a few hours? A few years? A few millenia? Much longer?
If you wish, give them the following estimates: Google says the mass of a proton is about kilograms. The wikipedia page on the “Observable Universe” says it has a mass of about kilograms.