When we test for the difference of two means, it makes a difference whether the samples are paired or independent. In this video Dr Nic clarifies the difference between the two types of data with two examples.
0:00 Introduction
0:29 Paired samples - example of before and after data
1:07 Independent samples - example of different treatments for different groups
1:40 Cat food example, paired
2:10 Cat food example, independent samples
2:30 Comparing data structures
2:58 Graph types
4:17 Credits and call to join

The accompanying video on analysis of paired data can be found here:    • T-test for difference of two means in...  

Video: Hypothesis test: t-test for difference of two means, independent samples    • Hypothesis testing: step-by-step, p-v...  

See https://creativemaths.net/videos/ for all of Dr Nic's videos organised by topic.