Ask a sample
Pick how many people to ask, then tap "Ask one sample". Each dot is one of the 5,000 people in a town. Orange dots would say yes to a question and blue dots would say no. You cannot ask everyone, so you ask a few, picked at random. How close does your sample get to the truth?
The town. Big dots are the people you asked.
Every sample so far. Each block is one sample, placed at its percentage of yes.
What's going on
In this town 1,800 of the 5,000 people would say yes. That is 36%, and it is the truth you are trying to find. A sample is the small group you actually ask. In a fair sample, every person in town has the same chance of being picked.
A fair sample is rarely exactly right, but it is right on average, and the misses are as likely to be too high as too low. Ask 200 samples of 50 and the pile sits around 36%, with most samples within about 10 points. Now choose 1,000 people: most samples land within about 3 points. Bigger samples make a narrower pile. To halve the width of the pile you need about four times as many people.
Now tick the lopsided switch. More people on the east side of this town say yes, and the lopsided sample only asks there. The whole pile slides away from the truth. Asking more people does not help: the pile gets narrower, but it is narrow around the wrong number. Who you ask matters more than how many you ask.
Sheets: Ask 50 people, learn about 5,000 and How can 1,000 people speak for a country?