Newsround has gone behind the scenes to find out what goes on at an animal testing laboratory.
Laura visited a lab in Oxfordshire, where mice are being used to try to work out a cure for glue ear - a problem which affects many children.
Using animals for experiments is a big issue, causing lots of debate.
People in favour of it say it's necessary to develop important medicines, but those against it say it's cruel.
Millions of animals are used in medical experiments across the UK every year.
Affects 85% of kids under 10
The middle bit of the ear fills with glue-like fluid instead of air
People with glue ear can't hear so well
In most cases, it clears without the need for treatment
If it carries on, sufferers might have an operation to cure it
The scientists at the lab in Oxfordshire are using the mice to try to work out more about glue ear, and whether it is passed from parents to children.
Mice don't look like humans, but there are similarities when it comes to looking inside their bodies.
Newsround also talked to animal rights campaigners. They want scientists to find other ways of making new medicines.
One said that testing "results in terrible suffering for millions of animals".
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