did-you-kno:

If you want someone to agree to a
request, using the phrase, ‘but you are
free to refuse,’ will double your chances.
While studies show it works equally well
no matter the type of request, how or why
it works remains unclear – but there are
theories that it puts people at ease
because it shows respect and doesn’t
make them feel like their freedom
to say no is being violated. Source Source 2 Source 3