You have 4 jars of pills. Each pill is a certain weight, except for contaminated pills contained in one jar, where each pill is weight + 1. How could you tell which jar had the contaminated pills in just one measurement? You have only spring weighting.
Assuming Regular pill is 1 weight...
Take 1 pill from Jar 1, take 2 pills from Jar 2, take 3 pills from Jar 3, and then take 4 pills from Jar 4, and then put them all on a scale together.
If the bad jar is Jar 1 then the weight will be 11 weight, if the bad jar is Jar 2 then weight will be 12 weight, bad jar 3 the weight is 13 weight, bad jar 4 the weight will be 14 weight.