This is a tiny particle, and is actually an electron!
HANG ON! Did we not say that radioactivity came from the nucleus? How can beta come from the nucleus - electrons are around the outside?
That's right, but things are more complicated than they seem. Beta particles do come from the nucleus.
Being an electron, beta particles are very small, and have a single negative charge. This makes it easier for them to pass through things.
Paper is not enough - it takes several millimetres of sheet metal - for example aluminium - to stop them normally.