They?

I think this is what happens in particle accelerators when particles with huge energies collide and their energy is transformed into exotic new particles.

I seem to do it quite fine when I eat too many calories. My mass increases quite predictably. But seriously, this is what the anything with a metabolism does, right?

Photosynthesis is a good example. See Jan Baptista van Helmont’s willow tree experiment.

Yep. Plants have a metabolism.

Actualy we do it every day. Mass increases as an object approaches the speed of light according to this formula .

@LuckyGuy I completely missed the velocity angle. E=MC2 after all.