Waterbeads are an excellent play resource to aid sensory stimulation and promote scientific exploration with preschoolers. They're also really good fun!
Waterbeads are tiny hard beads which when immersed in several litres of water, swell and absorb the water. They grow from 1mm across to the size of a pea, are soft and squidgy when touched, bouncy when dropped on the floor. Really bouncy! More commonly used in flower arrangements by florists, they are also a great resource for play, and are non-toxic (just make sure they don't taste-test them!).
I'd seen a few ideas on Pinterest involving lights and waterbeads; several used lightboxes and waterbeads, others used fairy lights. I'd also seen several ideas involving the plastic resuable eggs which are popular at Easter time for egg hunts and available from pound shops.
I bought some small submersible lights and soaked up a kaleidoscope of waterbeads. Into each plastic egg I placed a light and a handful of mixed colour beads, then put them with other beads and some loose water into a plastic tray for The Boy to explore, then turned the lights off and invited him in to play!
He was enthralled with the light shining through the waterbeads, the different colours that could be seen and the way the rainbow effect could be altered. As he's not one for messy play, this is a gentle nudge in the right direction for him with the beads feeling wet but not gloopy.
We discussed why the lights could shine through the waterbeads and I introduced the term 'translucent' to him, we then investigated what else the light could shine through and if coloured items altered the light. It's also a great activity for developing fine motor skills through opening and closing the eggs, twisting the lights on and picking up the water beads.
Safety note: Waterbeads swell up in water or equivalent liquid and they can grow quite large. This makes them incredibly bouncy, and easy to roll into corners to be discovered at a later date by a curious child. Please take care to prevent these being swallowed as they can be harmful (even though they are non-toxic).