I was doing some exercises the other day, when I came across this question in my book:
A proton weighs about 1.66 x 10-24 g and has a diameter of about 10-15 m. What is its density in g/cm3?
As you can see, a really simple and standard question, but... does it even make sense to say that a proton has a mass? And calculate its density?! If so, do I consider it a sphere?
It seems to me, subatomic particles are so small it doesn't really make sense to talk about mass, volume and density as if we were talking about... tennis balls!