Atoms that are unstable undergo radioactive decay,
Elements with half life(that's how we know that it decayed),
Atoms that release energy and particles(like alpha beta and gamma particles) and,
Elements that have turned into a new element (as in radioactive decay no. of protons can decrease)
And to be precise the elements that have neutron to proton ratio of more than 1 undergo radioactive decay, since the nuclear attraction gets all weird after that.
Organisms adapt to changes in their surroundings.
You can control time in a experiment EX: say you let a plant sit in the sun for 5 hours and then let another plant sit for an entire full 24 hours and see what's the effect well what you controlled there was the time and your dependent variable would be what ever happened to that plant. so you can control time and you can control speed if its an object that acquires you to exert force.
Cell membrane is what covers a cell