Triops are very clever, their eggs are designed not to all hatch out at once. This saves them all from dying if for example the pool gets filled by just a passing shower and would only last a week or less. If they had all hatched out, they'd all die. Some eggs will not hatch out in the pool until they receive a second dose of rain water. So by staggering their hatching rate in this way they maximise their survival. One reason why Triops have survived over 200 Million years! See Eggs and Babies article for more detail.