At this time of year, you often see little beetles like this one, on your windowsills.
This is a carpet beetle (Anthrenus verbasci). They are about 3mm long and they’re usually quite dopey and easy to catch. These beetles themselves do no damage: they feed on pollen. However, their larvae feed on natural fibres, including carpets, hence the name. They’ll also feed on any other natural materials, like your clothes, rugs, bedding, etc.
When you find a beetle, it could have come in from outside (e.g. through an open window, or quite often, on your laundry if it’s dried on the line), or it may have been living its larval life somewhere nearby. The best thing to do is to put the beetles outside again to try and stop it laying eggs in your house.
To try and avoid problems with the larvae in your house, make sure you vacuum thoroughly around the edges of your rooms, under your beds and on the stairs. Also, every now and then, shake out all your clothes and bedding etc. especially things that are kept in the airing cupboard.
Confusingly, the carpet beetle larvae are called woolly bears. That is also a common name for the caterpillar of the garden tiger moth (Arctia caja). The larvae of carpet beetles are small, only growing to about 7mm at most. They’re fairly inconspicuous, and if you do find one, they look a little like a hairy maggot:)
By contrast, the caterpillar of the garden tiger moth grows to around 7cm long – 10 times as big as the carpet beetle larvae. It is a lovely chocolatey colour on top, with ginger sides and long white hairs standing tall along its back. It has nothing to do with carpet beetles!