"It's to make the potion that keeps the horns from sprouting out of your head. That Jello you like so much? -- Toenails, all the way."
Of course, you don't have to tell him this right away, even though it's the truth.
“Tests were conducted, experiments performed, hypothesis proven or disproven, mistakes were made. I’m sorry Frank, but if we don’t continue these treatments and augmentations you will eventually revert to having armored scales, claws, and horns. And nobody likes horns as it interferes with wearing hats.”
I’d be a terrible parent to children of my own...